Porous structure

Abstract

Provided is a porous structure capable of further enhancing mechanical characteristics. A porous structure according to the present invention is a porous structure constituted of a plurality of tubes and comprising a first tube, which has a first space therein, and a second tube which has a second space therein. The second tube is provided on the first tube so as to intersect the first tube, and the first tube and the second tube are joined so that the first space and the second space communicate at an intersecting portion where the first tube and the second tube intersect.

Description

porous structure

[0001] The present invention relates to a porous structure composed of a plurality of tubes.

[0002] A structure composed of a plurality of tubes (hereinafter referred to as a "porous structure") is known. The porous structure has, for example, a structure in which a plurality of tubes extending in any direction are stacked in the thickness direction.

[0003] Patent Document 1 discloses a structure made of metal nanotubes. Patent Document 1 describes a method for producing metal nanotubes by coating the surface of a nanofiber with a metal layer and then removing the nanofiber by heating.

[0004] JP 2010-37592 A

[0005] The structure made of metal nanotubes of Patent Document 1 has room for improvement in terms of further improving mechanical properties such as strength and rigidity while ensuring sufficient flexibility.

[0006] Therefore, an object of the present invention is to solve the above problems by providing a porous structure that can improve mechanical properties while suppressing a decrease in flexibility.

[0007] A porous structure according to one aspect of the present invention is a porous structure composed of a plurality of tubes, comprising a first tube having a first space therein and a second tube having a second space therein, the second tube being disposed on the first tube so as to intersect with the first tube, and the first tube and the second tube being joined at the intersection where the first tube and the second tube intersect so that the first space and the second space are connected.

[0008] According to the present invention, it is possible to provide a porous structure that can improve mechanical properties while suppressing a decrease in flexibility.

[0009] 5A is a schematic perspective view of a porous structure according to an embodiment of the present invention. FIG. 1 is a schematic enlarged cross-sectional view taken along line II-II in FIG. 1. FIG. 3A is a schematic perspective view showing an example of a manufacturing method for the porous structure according to the embodiment. FIG. 3A is an enlarged cross-sectional view taken along line IIIC-IIIC in FIG. 3B. FIG. 3A is an enlarged cross-sectional view of region R2 in FIG. 3A. FIG. 4A is a schematic perspective view showing an example of a manufacturing method for the porous structure according to the embodiment. FIG. 4A is an enlarged cross-sectional view of region R2 in FIG. 4A. FIG. 5A is a schematic perspective view showing an example of a manufacturing method for the porous structure according to the embodiment. FIG. 5A is an enlarged cross-sectional view of region R1 in FIG. 5A. FIG. 5B is a cross-sectional view taken along line VC-VC in FIG. 5B. FIG. 5A is an enlarged cross-sectional view of region R2 in FIG. 5A. FIG. 6A is a schematic perspective view showing an example of a manufacturing method for the porous structure according to the embodiment. FIG. 6A is an enlarged cross-sectional view of region R1 in FIG. 6A. FIG. 6B is a cross-sectional view taken along line VIC-VIC in FIG. 6A. FIG. 6A is an enlarged cross-sectional view of region R2 in FIG. 6A. FIG. 6A is a diagram showing the results of analysis of the porous films of Examples 1 and 2 by X-ray diffraction (XRD). FIG. 6B is a diagram showing a scanning electron microscope (SEM) image of the top surface of the Cu porous film of Example 1. 11A is a diagram showing an SEM image of the upper surface of the Cu porous film of Example 1. FIG. 12A is a diagram showing an SEM image of the lower surface of the Cu porous film of Example 1. FIG. 12B is a diagram showing an SEM image of the lower surface of the Cu porous film of Example 1. FIG. 12C is a diagram showing an SEM image of the cross section of the Cu porous film of Example 1. FIG. 12D is a diagram showing an SEM image of the cross section of the Cu porous film of Example 1. FIG. 12E is a diagram showing an SEM image of the cross section of the Cu porous film of Example 1. FIG. 12F is a diagram showing an SEM image of the cross section of the Cu porous film of Example 1. FIG. 12G is a diagram showing an SEM image of the cross section of the Cu porous film of Example 1. FIG. 12H is a diagram showing an SEM image of the cross section of the Cu porous film of Example 1.

[0010] (Findings that Form the Basis of the Present Invention) The present inventors have conducted extensive research to further improve the mechanical properties of a porous structure while maintaining its flexibility, and have made the following findings.

[0011] Conventional porous structures such as those described in Patent Document 1 have relatively low strength and may be difficult to handle as a self-supporting structure. In response to this, for example, a configuration in which tubes are joined together to form a joint is considered in order to increase the strength and rigidity of the structure. However, it may be difficult to join the tubes more firmly. Furthermore, providing a joint between the tubes may reduce the flexibility of the porous structure. When flexibility is reduced, the tubes are more likely to break when the porous structure is bent, and desired properties may not be achieved. "Flexibility" refers to the flexibility required for substrates such as printed wiring substrates, and can be evaluated using a bending resistance test (JIS C5016) or the like.

[0012] Therefore, as a result of intensive research, the inventors have found that by joining two tubes at their intersection so that their internal spaces are in communication, it is possible to further improve mechanical properties while suppressing a decrease in flexibility. Based on this novel finding, the inventors have arrived at the following invention.

[0013] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. Note that the present disclosure is not limited to these embodiments. In addition, substantially identical components in the drawings are designated by the same reference numerals. For illustrative purposes, the dimensions of each element in the drawings may be exaggerated and are not necessarily drawn to scale.

[0014] Furthermore, for the sake of convenience, terms indicating directions such as "up," "down," "right," "left," and "side" are used below assuming a state during normal use, but are not intended to limit the state of use of the porous structure according to the present disclosure. Furthermore, "perpendicular" or "substantially perpendicular" includes the case where the porous structure is substantially perpendicular, taking into account a realistic range of variation. "Parallel" or "substantially parallel" includes the case where the porous structure is substantially parallel, taking into account a realistic range of variation.

[0015] In the drawings described below, for reference, mutually orthogonal X-axis, Y-axis, and Z-axis are schematically shown. The Z-axis corresponds to the thickness direction of the porous structure.

[0016] <<Embodiment>> Fig. 1 is a schematic perspective view of a porous structure according to an embodiment of the present invention. Fig. 1 also shows a schematic enlarged perspective view of a portion of the porous structure. Fig. 2 is a schematic cross-sectional view taken along line II-II in Fig. 1.

[0017] As shown in Fig. 1, the porous structure 1 of this embodiment has an upper surface 11 and a lower surface 12 that are spaced apart from each other in the thickness direction (Z direction). In the example shown in Fig. 1, the porous structure 1 is a rectangular porous membrane. The thickness t of the porous structure 1 can be selected appropriately depending on the application, the type of tube, the formation method, etc. As an example, the thickness t is 1 µm or more and 500 µm or less.

[0018] The porous structure 1 includes a plurality of tubes 2. The porous structure 1 may be a nonwoven fabric-like structure in which the plurality of tubes 2 are randomly oriented, or a woven fabric-like structure. In this embodiment, the porous structure 1 is formed by stacking a plurality of tubes 2 extending in any direction intersecting the Z direction in the Z direction.

[0019] In the porous structure 1 of this embodiment, pores are formed between adjacent tubes 2 and in the internal space of each tube 2. The porosity of the porous structure 1 is not particularly limited, but is, for example, 70% to 95%. In this specification, the term "porous structure" refers to a structure composed of a plurality of tubes and having pores at least between the tubes. The porous structure may further include components other than tubes. In FIG. 1, the porous structure 1 is a porous membrane, but the shape of the porous structure is not limited to a membrane.

[0020] The tube 2 is a metal tube made of a metal such as Cu, Ni, Pd, Sn, Au, Ag, or Pt. The material of the tube 2 is not limited to metal, but may be an alloy, a metal oxide, or the like. Examples of metal oxides include nickel oxide, copper oxide, tin oxide, and silver oxide. The tube 2 may have a single-layer structure or a layered structure including two or more films with different compositions.

[0021] The tube 2 is, for example, cylindrical or approximately cylindrical. The cross-sectional shape of the tube 2 is not limited to circular or approximately circular, but may also be elliptical or approximately elliptical. The inner diameter of the tube 2 may be, for example, 0.05 μm to 5 μm. The thickness u of the tube 2 may be, for example, 0.01 μm to 10 μm. The "inner diameter of the tube" refers to the maximum width of the internal space of the tube in a cross section perpendicular to the extension direction of the tube.

[0022] The multiple tubes 2 include a first tube 21 and a second tube 22. The first tube 21 has a first space 31 therein. The second tube 22 has a second space 32 therein. In this embodiment, the first space 31 and the second space 32 are hollow (e.g., air layers). In this specification, a tube structure with a hollow interior may be referred to as a "hollow structure."

[0023] The second tube 22 is provided on the first tube 21 so as to intersect with the first tube 21. At the intersection 5 where the two tubes 21, 22 intersect with each other, the first tube 21 and the second tube 22 are joined to each other. In Figures 1 and 2, an example will be described in which the two tubes 21, 22 are stacked at the intersection 5 substantially in the Z direction (i.e., the thickness direction of the porous structure 1). Note that the arrangement of the two tubes constituting the intersection is not limited to the example shown in the figures.

[0024] 2 , at the intersection 5, the first tube 21 and the second tube 22 are joined so that the first space 31 and the second space 32 communicate with each other, forming a joint 201. The inner surfaces of the first tube 21 and the second tube 22 define a connected space including the first space 31 and the second space 32. When viewed along the Z direction, the joint surface 2j between the first tube 21 and the second tube 22 is, for example, an annular surface positioned so as to surround a communication portion 301 where the first space 31 and the second space 32 communicate with each other.

[0025] In this embodiment, in a cross section at the intersection 5 intersecting the extending direction of the first tube 21 and the extending direction of the second tube 22, the joined first tube 21 and second tube 22 are joined so as to have a constricted portion (narrow portion) at a communicating portion 301 between the first space 31 and the second space 32. The width w3 of the communicating portion 301 is smaller than the width w1 of the first space 31 and the width w2 of the second space 32. The widths w1 to w3 are widths along a direction perpendicular to the stacking direction of the tubes 21 and 22 (here, the Z direction). The width w3 of the communicating portion 301 may be smaller than the inner diameters of the first tube 21 and the second tube 22, for example.

[0026] In the cross section shown in Figure 2, the first tube 21 and the second tube 22 extend in an arc that is greater than 180° and less than 360°, and the ends of these arc-shaped portions join together to form a joint surface 2j. The "arc-shaped" is not limited to a circular arc or a nearly circular arc, but may also be an elliptical arc or a nearly elliptical arc. The center of an imaginary circle (ellipse) that includes the arc-shaped portion of the first tube 21 is located on the -Z side of the communicating portion 301, and the center of an imaginary circle (ellipse) that includes the arc-shaped portion of the second tube 22 is located on the +Z side of the communicating portion 301.

[0027] Although Fig. 2 illustrates a joint 201 having a constricted portion, the joint does not have to have a constricted portion. Furthermore, Fig. 2 illustrates a joint 201 between two tubes 2, but even at an intersection where three or more tubes intersect, these tubes 2 can be joined so that the internal spaces of the tubes 2 communicate with each other (see joint 202 shown in Fig. 10). Furthermore, the porous structure of this embodiment may include a mixture of multiple joints with different numbers of joined tubes and different cross-sectional shapes of the intersections.

[0028] (Method for manufacturing porous structure) Next, an example of a method for manufacturing the porous structure of this embodiment will be described with reference to the drawings. Here, a method for manufacturing a metal porous membrane composed of metal (e.g., Cu) tubes will be described as an example of the porous structure.

[0029] Figures 3A, 4A, 5A, and 6A are schematic perspective views illustrating an example of a manufacturing method for a porous structure according to this embodiment. Figures 3B and 3C are an enlarged perspective view and an enlarged cross-sectional view, respectively, of region R1 in Figure 3A. Figure 3D is an enlarged cross-sectional view of region R2 in Figure 3A. Figures 4B and 4C are enlarged cross-sectional views of regions R1 and R2 in Figure 4A. Figures 5B and 5C are an enlarged perspective view and an enlarged cross-sectional view, respectively, of region R1 in Figure 5A. Figure 5D is an enlarged cross-sectional view of region R2 in Figure 5A. Figures 6B and 6C are an enlarged perspective view and an enlarged cross-sectional view, respectively, of region R1 in Figure 6A. Figure 6D is an enlarged cross-sectional view of region R2 in Figure 6A.

[0030] <Formation of Fiber Structure (Fiber Membrane): FIGS. 3A to 3D> First, a 20 wt % polymethyl methacrylate resin (PMMA) solution is prepared using DMF (N,N-dimethylformamide) as a solvent. The obtained PMMA solution is used as a spinning solution. Next, as shown in FIGS. 3A to 3D, the spinning solution (PMMA solution) is used to spin fibers 4 made of PMMA onto the surface of a substrate 8 by electrospinning. In this way, a fiber membrane (nonwoven fabric membrane) 7 made of a plurality of fibers 4 is formed on the substrate 8 as a fiber structure.

[0031] Here, a Cu foil with a thickness of 0.03 mm is used as the substrate 8. The electrospinning conditions are set, for example, as follows: Applied voltage: 23 kV Distance between the nozzle and the substrate: 14 cm Film formation time: Adjusted so that the thickness of the fiber film is approximately 100 μm The diameter (fiber diameter) of the fiber 4 is approximately 1000 nm to 2000 nm. The fiber diameter can be controlled, for example, within a range of several tens of nm to several thousands of nm, depending on the solution concentration, conductivity, etc.

[0032] 3B and 3C, in a region R1 located inside the fiber membrane 7, two fibers 4 (hereinafter referred to as a "first fiber 41" and a "second fiber 42") cross each other and are stacked, for example, in the thickness direction (Z direction) of the fiber membrane 7. As shown in Fig. 3D, in a region R2 located near the surface of the substrate 8, a fiber 4 (hereinafter referred to as a "third fiber 43") is arranged on the surface of the substrate 8.

[0033] 4A to 4C, the fiber membrane 7 on the substrate 8 is subjected to a first heat treatment to fuse the fibers together. This results in a fused fiber structure (fused fiber membrane) 7a. The heat treatment temperature is set to a temperature equal to or higher than the glass transition temperature Tg of PMMA (e.g., 80°C to 150°C).

[0034] As shown in Fig. 4B , in region R1 located inside the welded fiber membrane 7a, the first fiber 41 and the second fiber 42 are welded to each other at their intersections and integrated. In the example shown in Fig. 4B , the width of the welded portion of the integrated fibers is smaller than the width of the fibers 41 and 42. As shown in Fig. 4C , in region R2 near the substrate 8 of the welded fiber membrane 7a, the third fiber 43 is welded to the surface of the substrate 8. In this example, the third fiber 43 is flattened and deformed to have a substantially flat surface portion 43p facing the substrate 8.

[0035] <Metal Coating: Figs. 5A to 5D> Next, for example, O ２The welded fiber membrane 7a is subjected to a hydrophilic treatment using plasma, and then the hydrophilic welded fiber membrane 7a is subjected to a surface treatment using a surface charge adjusting solution, a catalyst applying solution (alkaline catalyst), and a Pd reducing solution as a pretreatment for plating.

[0036] Next, as shown in Figures 5A to 5D, the welded fiber membrane 7a is coated with metal, for example, by electroless plating, to form a metal-coated fiber structure (here, a metal-coated fiber membrane) 7b. In the metal-coated fiber membrane 7b, the tube 2 is formed by a metal membrane that coats the circumferential surface of each fiber 4. The thickness of the metal membrane is approximately 100 nm to 300 nm. The thickness of the metal membrane can be controlled within a range of, for example, 1 nm to several thousand nm by adjusting the plating conditions (plating time).

[0037] 5B and 5C , in region R1 located inside the metal-coated fiber membrane 7b, metal films that become the first tube 21 and the second tube 22 are formed around the circumferential surfaces of the first fiber 41 and the second fiber 42 that are welded together. The metal films covering the first fiber 41 and the second fiber 42 are connected to each other and integrated in a gap near the welded portions of these fibers 41 and 42, forming a joint 201.

[0038] 5D , in region R2 of the metal-coated fiber membrane 7b near the substrate 8, the plating solution also penetrates between the third fiber 43 and the substrate 8, forming a metal film on the flat portion 43p of the third fiber 43. As a result, a metal film that becomes the third tube 23 is formed so as to cover the circumferential surface of the third fiber 43. The third tube 23 has a flat portion 23p on the substrate 8 side, reflecting the shape of the third fiber 43. The flat portion 23p is the surface of the portion of the third tube 23 that is located on the flat portion 43p of the third fiber 43.

[0039] <Removal of Substrate> Next, the substrate 8 is removed from the metal-coated fiber membrane 7b as shown by the arrow in Fig. 5D. For example, the substrate 8 on which the metal-coated fiber membrane 7b is formed is dried using an oven, whereby the substrate (Cu foil) 8 can be removed.

[0040] <Pyrolysis of Fibers: Figures 6A to 6D> Next, a second heat treatment is performed on the metal-coated fiber membrane 7b, thereby pyrolyzing the fibers 4 in each tube 2. As a result, as shown in Figures 6A to 6D, the fibers 41 to 43 in the tubes 21 to 23 are removed, and a porous structure (here, a metal porous membrane) 1 composed of hollow tubes is obtained.

[0041] Here, the second heat treatment is performed in a nitrogen atmosphere at a heat treatment temperature of 500° C. for 2 hours. ２ ) Atmosphere, N ２ / H ２ By carrying out the treatment in a reducing atmosphere such as a mixed atmosphere, it is possible to thermally decompose the fiber 4 while suppressing oxidation of the metal film that constitutes the tube 2 (which becomes the tube wall).

[0042] The upper surface 11 of the porous structure 1 is mainly composed of the peripheral surfaces of multiple tubes 2 randomly arranged in the uppermost layer of the porous structure 1. For this reason, microscopic irregularities are likely to form on the upper surface 11, and the surface roughness is likely to increase. On the other hand, as shown in FIG. 6D , the lower surface 12 of the porous structure 1 (the surface from which the substrate has been peeled off) can be defined by the flat portion 23p of the third tube 23. Therefore, the lower surface 12 can be made smoother than the upper surface 11 of the porous structure 1. Furthermore, the density of the tubes 2 (or the density of the metal (Cu) that forms the tube walls) on the lower surface 12 can be made higher than on the upper surface 11.

[0043] The materials of the fiber 4 and the metal film are not particularly limited. The fiber 4 material usable in the above method may be any material that can be welded at a first temperature and thermally decomposed at a second temperature. The second temperature is higher than the first temperature and lower than the melting point of the material (e.g., Cu) of the metal film that will become the tube 2. In addition to PMMA, other thermoplastic resins that can be used for the fiber 4 include polyimide, polyamide, polyamideimide, polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyvinyl acetate, polytetrafluoroethylene, liquid crystal polymer, polyphenylene sulfide, polyether ether ketone, polysulfone, polyethersulfone, polycarbonate, modified polyphenylene ether, polybutylene terephthalate, polyethylene terephthalate, polyacetal, polylactic acid, polyvinyl alcohol, ABS resin, polyvinylidene fluoride, polyethylene oxide, polyethylene glycol, and polyurethane. The metal film material is not limited to Cu, and may be Ni, Pd, Sn, Au, Ag, Pt, or the like.

[0044] According to the above method, a porous structure composed of multiple tubes can be more easily manufactured by utilizing the shape of the welded fiber structure. In the above method, the porous structure is formed by electroless plating on a fiber structure whose strength has been increased by welding. Therefore, gas generated during electroless plating is less likely to cause swelling or wrinkles in the porous structure or breakage of the fibers or metal film (tubes). Therefore, the surface roughness of the porous structure 1 can be reduced compared to conventional porous structures such as those described in Patent Document 1. Furthermore, in the above method, joints 201 of the tubes 2 are formed at the welded portions between the fibers. Therefore, more joints 201 can be formed while maintaining porosity. Therefore, the strength of the porous structure 1 can be increased.

[0045] The method for manufacturing the porous structure of this embodiment is not limited to the method exemplified in FIGS. 3A to 6D.

[0046] In the above method, the second heat treatment for pyrolyzing the fiber is performed in a reducing atmosphere. However, the second heat treatment may be performed in an oxidizing atmosphere such as air. This causes the fiber inside the tube to pyrolyze and oxidizes the metal film that constitutes the tube (the tube wall). This results in a porous structure composed of hollow metal oxide tubes.

[0047] In the above method, electrospinning is used to form the fiber structure, but alternatively, melt-blowing, flash spinning, centrifugal spinning, melt spinning, etc. may be used. In addition, in the above method, the fibers are fused together by heating (first heat treatment), but alternatively, the fibers may be joined together by light irradiation.

[0048] In the above method, the fiber membrane is surface-treated and a catalyst is applied before electroless plating. However, as described in Patent Document 1, electroless plating may also be performed on resin fibers mixed with a catalyst. However, the method of Patent Document 1 may make it difficult to form a uniform plating film around the fibers. This can result in the formation of micropores or cracks in the film, making it appear as two layers (see Figure 1 of Patent Document 1). In contrast, the above method allows the catalyst to be applied uniformly to the surface of each surface-treated fiber, resulting in the formation of a denser Cu plating film. The surface of the Cu plating film (the surface that will become the outer surface of the tube) is smooth or approximately smooth (see Figures 8B and 9B described below). In this specification, the term "smooth or approximately smooth outer surface of the tube" refers to, for example, a state in which defects, pores, micropores, cracks, etc. are not formed or are approximately not formed on the outer surface of the tube.

[0049] In the above method, the welded fiber structure is coated with a metal, but it may also be coated with a material other than metal (e.g., titanium oxide, silicon dioxide). Furthermore, the coating method for the welded fiber structure is not limited to electroless plating, and a sol-gel method or the like can also be used.

[0050] In the above method, the surface of the porous structure from which the substrate is removed (the lower surface) is made smoother than the upper surface by utilizing the welding of the fiber to the substrate (see Fig. 6D). Note that depending on the type and shape of the substrate and the welding method of the fiber, the fiber may not be welded to the substrate, resulting in the upper and lower surfaces of the porous structure having the same surface roughness.

[0051] In the above method, the shape of the fused fiber structure is used to produce a metal porous structure, but it is not necessary to use a fiber membrane. For example, a porous structure having a structure such as that shown in Figures 1 and 2 can be produced using a 3D printer.

[0052] (Effect) In the porous structure 1 of this embodiment, as shown in Figures 1 and 2, at the intersection 5 where the first tube 21 and the second tube 22 intersect, a joint 201 is formed where the first tube 21 and the second tube 22 are joined so that the first space 31 of the first tube 21 and the second space 32 of the second tube 22 are connected.

[0053] The above-described configuration can improve the mechanical properties of the porous structure 1 while suppressing a decrease in flexibility. Specifically, by forming a joint 201 between the first tube 21 and the second tube 22, the rigidity and mechanical strength of the porous structure 1 can be increased. At the joint 201, for example, a joint surface 2j is formed so as to surround a communication portion 301 between the first space 31 and the second space 32. This allows for a larger joint area than, for example, when only the outer surfaces of the two tubes are joined together (see FIG. 13 ). This increases the joint strength of the joint 201, thereby achieving higher mechanical properties. Furthermore, because the first space 31 and the second space 32 communicate with each other at the joint 201 to form a single space (connected space), a decrease in flexibility due to the formation of the joint 201 can be suppressed compared to, for example, when the spaces within the tubes are separated from each other (see FIG. 13 ).

[0054] In the porous structure 1 of this embodiment, the first space 31 of the first tube 21 and the second space 32 of the second tube 22 are hollow. With this configuration, it is possible to increase the porosity of the porous structure 1. Furthermore, it is possible to reduce the weight of the porous structure 1.

[0055] In the porous structure 1 of this embodiment, in a cross section at the intersection 5 that intersects with the extending direction of the first tube 21 and the extending direction of the second tube 22, the first tube 21 and the second tube 22 are joined so as to have a constricted portion at the communicating portion 301 between the first space 31 and the second space 32. With this configuration, the strength and rigidity of the porous structure 1 can be increased while minimizing the decrease in flexibility and porosity compared to a structure without a constricted portion.

[0056] In this embodiment, the first tube and the second tube are metal tubes. With this configuration, the joints 201 between the metal tubes can increase the number of current paths, thereby lowering the electrical resistance of the porous structure. Furthermore, the inherent properties of metal (malleability and ductility) can further increase the mechanical strength. Furthermore, the thermal conductivity of the porous structure can be increased. Therefore, a metal porous structure with high thermal conductivity, low resistance, and excellent mechanical properties can be provided.

[0057] The thickness t of the porous structure 1 of this embodiment is, for example, 10 μm or more. This increases the strength of the porous structure 1, making it possible to handle the porous structure 1 as a free-standing structure (free-standing membrane). On the other hand, from the viewpoint of ensuring flexibility, the thickness t of the porous structure 1 may be, for example, 1 mm or less.

[0058] The porous structure 1 of this embodiment includes a plurality of bonding portions 201. The bonding portions 201 may be arranged randomly or regularly inside the porous structure 1. This can more effectively improve the mechanical properties of the porous structure 1.

[0059] In the porous structure 1 of this embodiment, for example, the surface roughness of the lower surface 12 can be made smaller than the surface roughness of the upper surface 11. With this configuration, the porous structure 1 can be more easily bonded to another member at the lower surface 12 of the porous structure 1.

[0060] In the porous structure 1 of this embodiment, for example, the tubes 2 are arranged more densely on the lower surface 12 than on the upper surface 11. With this configuration, the porous structure 1 can be more easily joined to other members at the lower surface 12 of the porous structure 1. Furthermore, when the porous structure 1 is a metal porous structure, the electrical resistance of the connection can be further reduced by electrically connecting the porous structure 1 to other members at the lower surface 12. Meanwhile, since the tubes 2 are arranged more sparsely on the upper surface 11 than on the lower surface 12, the flexibility of the porous structure 1 can be ensured.

[0061] In the porous structure 1 of this embodiment, as shown in Fig. 6D, the lower surface 12 of the porous structure 1 can be defined by the flat portion 23p of the tube 23 located nearby. With this configuration, the lower surface 12 of the porous structure 1 can be made smoother. Therefore, the lower surface 12 of the porous structure 1 can be easily joined to another member.

[0062] The porous structure 1 of this embodiment is manufactured by the method described with reference to Figures 3A to 6D, for example, and therefore blistering and wrinkles are suppressed compared to conventional porous structures, and surface roughness is reduced. This makes it possible to suppress deterioration of the properties (electrical properties, heat transport properties, etc.) of the porous structure 1 due to surface roughness. Furthermore, when the porous structure 1 is a metal porous structure, it is possible to suppress high resistance due to surface roughness.

[0063] When a conventional porous structure is used as a wick for, for example, a heat pipe or a vapor chamber, the surface roughness of the porous structure is large, making it difficult for the liquid to spread evenly on the wick, which may result in reduced heat conduction efficiency and cooling function. Furthermore, the pressure loss of the gas may increase, potentially reducing heat transfer efficiency. In contrast, when the porous structure 1 of this embodiment is used, the surface roughness is small, making it possible to achieve a wick with high heat transport properties.

[0064] The arithmetic mean roughness Ra of the porous structure 1 is, for example, 5 μm or less. Alternatively, the maximum height roughness Rz of the porous structure 1 is, for example, 20 μm or less. This makes it possible to more effectively suppress deterioration of characteristics due to surface roughness.

[0065] In the porous structure 1, the outer surfaces of the first tube 21 and the second tube 22 are smooth or approximately smooth. With this configuration, the properties (electrical properties, heat transport properties, etc.) of the porous structure 1 can be further improved.

[0066] Examples and Comparative Examples Hereinafter, the methods for producing, evaluation methods, and evaluation results of the porous membranes of Examples 1 and 2 and Comparative Examples will be described.

[0067] Example 1 A porous Cu film made of Cu tubes was produced by the method described with reference to FIGS. 3A to 6D.

[0068] Example 2 A porous copper oxide (CuO) film composed of CuO tubes was fabricated in the same manner as in Example 1, except that the second heat treatment for pyrolyzing the fiber was performed in an oxidizing atmosphere (here, air atmosphere) instead of a reducing atmosphere.

[0069] Comparative Example: A Cu porous film composed of Cu tubes was fabricated using the same method as in Example 1, except that the first heat treatment for fusing the fibers was not performed. In the comparative example, Cu films that would become Cu tubes were formed so as to cover each of the separated (unfused) fibers. At the intersection of two fibers close to each other, the outer surfaces of the Cu films (Cu tubes) covering these fibers can be bonded to each other, as illustrated in FIG. 13 . In addition, in the comparative example, the fibers near the substrate are not fused to the substrate, and therefore, a flat portion, such as that shown in FIG. 6D , is not formed in the Cu film covering the fibers near the substrate.

[0070] The methods for producing the porous membranes of Examples 1 and 2 and the comparative example are summarized in Table 1.

[0071] <Qualitative Analysis of Porous Film> Qualitative analysis of the porous films of Examples 1 and 2 was carried out by X-ray diffraction (XRD) using an X-ray diffractometer (MiniFlex600 manufactured by Rigaku Corporation).

[0072] The results of the XRD analysis are shown in Figure 7 and Table 1. As shown in Figure 7, a diffraction peak attributed to Cu was observed in Example 1, confirming that the porous film of Example 1 was a Cu porous film. In Example 1, the second heat treatment (thermal decomposition of the PMMA fiber) was performed in a reducing atmosphere, which is thought to have suppressed oxidation of the Cu tube. Although not shown, in the comparative example, the second heat treatment was performed in the same manner as in Example 1, and therefore the porous film of the comparative example is also a Cu porous film.

[0073] In Example 2, a diffraction peak attributed to CuO (copper (II) oxide) was observed, confirming that the metal porous membrane of Example 2 was a CuO porous membrane. This is thought to be because, in Example 2, the second heat treatment was performed in an oxidizing atmosphere (air atmosphere), which resulted in the PMMA fiber being thermally decomposed and the Cu membrane being oxidized.

[0074] <Observation of Microstructure of Porous Film> Observation Method (a) Observation of Surface Microstructure: SEM The upper and lower surfaces of the Cu porous film of Example 1 were observed using a scanning electron microscope (S-4800 manufactured by Hitachi High-Technologies Corporation).

[0075] (b) Cross-sectional microstructure observation: SEM, EDX The cross-section of the Cu porous film of Example 1 was observed by the following method. After the Cu porous film of Example 1 was solidified with resin, the cross-section was polished so that the Cu porous film was exposed. Then, ion milling was performed using a flat milling device IM-3000 manufactured by Hitachi High-Technologies Corporation to obtain a sample for cross-sectional observation. The cross-sectional observation sample obtained by the above method was observed using a scanning electron microscope (S-4800 manufactured by Hitachi High-Technologies Corporation).

[0076] The above-mentioned observation sample was also observed using a scanning electron microscope (S-4800 manufactured by Hitachi High-Technologies Corporation, accelerating voltage 15 kV, 1 k to 20 k magnification).

[0077] Observation Results FIG. 8A is a scanning electron microscope (SEM) image (acceleration voltage: 15 kV, 500x magnification) of the upper surface of the Cu porous film of Example 1. FIG. 8B is a SEM image (acceleration voltage: 15 kV, 5kx magnification) of the upper surface of the Cu porous film of Example 1. FIG. 9A is a SEM image (acceleration voltage: 5 kV, 500x magnification) of the lower surface of the Cu porous film of Example 1. FIG. 9B is a SEM image (acceleration voltage: 5 kV, 5kx magnification) of the lower surface of the Cu porous film of Example 1. From the SEM images shown in FIGS. 8A to 9B, it can be confirmed that the surface of the Cu porous film of Example 1 is smooth, and no wrinkles, bulges, breaks, etc. are visible.

[0078] 8B and 9B, it can be seen that the Cu tubes are made of a dense film and that the outer surfaces of the Cu tubes are smooth or nearly smooth. As described above, in Example 1, a catalyst is uniformly applied to the surface of each fiber that has been subjected to surface treatment, thereby performing Cu coating (plating), which is thought to be because the formation of pores, defects, micropores, cracks, etc. in the Cu film that becomes the Cu tubes is suppressed.

[0079] Fig. 10 is a diagram showing an SEM image of the fracture surface of the Cu porous membrane of Example 1. The SEM image shown in Fig. 10 reveals that the Cu porous membrane of Example 1 has a plurality of joints 201 and 202 in which two or more tubes are joined so that their internal spaces communicate with each other.

[0080] 11A is a diagram showing an SEM image of a cross section of the Cu porous membrane of Example 1. FIGS. 11B and 11C are enlarged schematic views of regions XIB and XIC near the top and bottom surfaces of the SEM image of FIG. 11A, respectively. In the SEM image, the region visible by contrast with the resin used for resin solidification corresponds to the cross-sectional image of the Cu porous membrane. Therefore, the thickness of the Cu porous membrane, Cu density (density of the Cu tubes), and the like can be confirmed from the SEM image.

[0081] The SEM image shown in Figure 11A shows that the thickness t of the Cu porous membrane of Example 1 is approximately 100 µm. It can also be seen that the Cu membrane is formed substantially uniformly in the thickness direction of the porous membrane. This is thought to be because the surface treatment of the fiber membrane before Cu coating (Cu electroless plating) made the fibers hydrophilic, enhancing their impregnation with the plating solution.

[0082] 11A to 11C, it is confirmed that in the Cu porous film of Example 1, the lower surface 12 is smoother than the upper surface 11 and that the Cu density is higher on the lower surface 12 than on the upper surface 11.

[0083] Although not shown in the figure, the thickness of the Cu porous film, Cu density, etc. can also be confirmed from a backscattered electron image and an element mapping image (here, a Cu mapping image) obtained by energy dispersive X-ray spectroscopy (EDX).

[0084] <Surface Roughness Measurement> The surface roughness of the Cu porous films of Example 1 and Comparative Example was measured. Here, the arithmetic mean roughness (Ra) and maximum height roughness (Rz) in accordance with JIS B0601-2001 were calculated as the surface roughness using the following method. A 3D shape measuring instrument (VR-5200 manufactured by Keyence Corporation) was used to photograph the surface of the Cu porous film at 25x magnification, and then analysis was performed in line roughness measurement mode using the attached analysis application. Line roughness was measured at 10 arbitrary locations within the measurement area (approximately 12 mm x approximately 9 mm). The cutoff value λc of the high-pass filter was set to 0.8 mm, and a roughness curve was obtained. The average value of the measurement results obtained at the 10 locations was calculated. The results are shown in Table 2.

[0085] As shown in Table 2, the surface roughness of the Cu porous film of Example 1 is smaller than that of the Cu porous film of the comparative example. Furthermore, from the SEM images shown in Figures 8A to 9B, it can be seen that in Example 1, no swelling, wrinkles, or broken tubes were visible in the Cu porous film, and the surface of the Cu porous film was smooth. The reason for this is thought to be as follows.

[0086] In the comparative example, the fiber membrane before Cu coating was performed was in a state where multiple fibers were stacked and separated from each other, and did not have sufficient strength. Therefore, the gas generated during electroless plating easily caused the porous membrane to swell, wrinkle, or break. Fiber breakage also easily caused breakage or disconnection of the Cu membrane (Cu tube). As a result, the surface roughness of the Cu porous membrane increased. Note that wrinkles, tube disconnections, and other conditions can be confirmed, for example, from SEM images described in Patent Document 1.

[0087] In contrast, in Example 1, the fiber membrane before Cu coating was a fused fiber membrane in which the fibers were fused (bonded) together. Because the membrane strength was increased by welding, the Cu porous membrane was less likely to bulge, wrinkle, or break the fibers or tubes due to gases generated during electroless plating. As a result, the surface roughness of the Cu porous membrane was significantly improved compared to the comparative example.

[0088] Furthermore, in the porous film of Example 1, the surface roughness of the surface (lower surface) 12 from which the substrate was peeled off is smaller than the surface roughness of the upper surface 11. In other words, the lower surface 12 is smoother. This is thought to be because, as can be seen from Figures 11A to 11C, Cu is present more densely on the lower surface 12 than on the upper surface 11, and the upper surface 11 is composed of the circumferential surfaces of the randomly stacked tubes 2, while the lower surface 12 is defined by the flat surfaces of the tubes 2 (see Figures 5D and 6D).

[0089] (Modification) The porous structure of this embodiment may be a composite of a plurality of tubes and a component other than the tubes.

[0090] Fig. 12A is a schematic perspective view showing a part of a porous structure of a modified example, and Fig. 12B is a schematic cross-sectional view taken along line XIIB-XIIB in Fig. 12A.

[0091] 12A and 12B , in the porous structure 1a of this modification, first fibers 41 are disposed in the first spaces 31 of the first tubes 21, and second fibers 42 are disposed in the second spaces 32 of the second tubes 22. The fibers 4 are, for example, resin fibers. By disposing the fibers 4 in the internal spaces of the plurality of tubes 2 in this manner, the mechanical properties can be improved compared to the porous structure 1 shown in FIGS. 1 and 2 .

[0092] In the illustrated example, the first fibers 41 are filled in the first space 31, and the second fibers 42 are filled in the second space 32. The first fibers 41 and the second fibers 42 are joined to each other at the intersections 5. This configuration can further improve the mechanical properties of the porous structure 1a, thereby enhancing handleability. Furthermore, the metal film constituting the tube 2 can be made thinner while maintaining the mechanical properties. Furthermore, by using a material for the fibers 4 that is more flexible (less rigid) than the material for the tube 2, the flexibility of the porous structure 1a can be enhanced.

[0093] The porous structure 1a of this modification may be manufactured by a method similar to that of the porous structure 1 shown in FIGS. 1 and 2 (see FIGS. 3A to 5D), except that the fiber is not pyrolyzed (second heat treatment). In this modification, since the fiber does not need to be pyrolyzed, there is a high degree of freedom in the selection of the fiber material. The fiber 4 applicable to this modification is, for example, an organic fiber containing an organic material. The fiber 4 is not limited to an organic fiber, but may also be an inorganic fiber containing an inorganic material or an organic-inorganic composite fiber containing an organic-inorganic composite material.

[0094] The inner space of the tube 2 does not have to be completely filled with the fibers 4, and a gap (air layer) may exist between the fibers and the inner surface of the tube. The presence of the gap can further enhance flexibility. For example, such a structure can be produced by using a fiber having an inner layer and an outer layer and pyrolyzing only the outer layer or the inner layer of the fiber.

[0095] The present invention is not limited to the above-described embodiment, and design modifications are possible within the scope of the gist of the present invention.

[0096] The above description can also be expressed as follows.

[0097] The porous structure of the first embodiment is a porous structure composed of a plurality of tubes, and comprises a first tube having a first space therein and a second tube having a second space therein, the second tube being provided on the first tube so as to intersect with the first tube, and the first tube and the second tube being joined at the intersection where the first tube and the second tube intersect so that the first space and the second space are connected.

[0098] A second aspect of the porous structure is the porous structure of the first aspect, wherein the first space and the second space are hollow.

[0099] The porous structure of the third aspect is the porous structure of the first or second aspect, wherein in a cross section at the intersection that intersects with the extension direction of the first tube and the extension direction of the second tube, the first tube and the second tube are joined so as to have a constricted portion at the part where the first space and the second space are connected.

[0100] A fourth aspect of the porous structure is the porous structure of any one of the first to third aspects, wherein the first tube and the second tube are metal tubes.

[0101] A porous structure of a fifth aspect is the porous structure of any one of the first to fourth aspects, wherein the porous structure has a first surface and a second surface arranged at a distance from each other in the thickness direction, and the surface roughness of the first surface is greater than the surface roughness of the second surface.

[0102] A sixth aspect of the porous structure is the porous structure of the fifth aspect, wherein the plurality of tubes are arranged more densely on the second surface than on the first surface.

[0103] A seventh aspect of the porous structure is the porous structure of the fifth or sixth aspect, wherein the tube located near the second surface has a flat portion that defines the second surface.

[0104] The porous structure of an eighth aspect is the porous structure of any one of the first to seventh aspects, wherein the porous structure has an arithmetic mean roughness Ra of 5 μm or less, or a maximum height roughness Rz of 20 μm or less.

[0105] A porous structure of a ninth aspect is the porous structure of any one of the first to eighth aspects, wherein a first fiber is arranged in the first space, and a second fiber is arranged in the second space.

[0106] A porous structure of a tenth aspect is the porous structure of the ninth aspect, wherein the first fiber is filled in the first space, the second fiber is filled in the second space, and the first fiber and the second fiber are joined at the intersection.

[0107] The porous structure of an eleventh aspect is the porous structure of any one of the first to tenth aspects, wherein the outer surfaces of the first tube and the second tube are smooth or approximately smooth.

[0108] The porous structure of the present invention has excellent mechanical properties and sufficient flexibility, and therefore can be used as a variety of functional substrates in a wide range of fields, including, for example, wicks for heat pipes, electrodes for various batteries, sanitary products, and medical devices.

[0109] DESCRIPTION OF SYMBOLS 1 Porous structure 2 Tube 2j Joint surface 4 Fiber 5 Intersection 7 Fiber membrane 7a Welded fiber membrane 7b Metal coated fiber membrane 8 Substrate 11 Upper surface 12 Lower surface 21 First tube 22 Second tube 23 Third tube 23p Flat surface 31 First space 32 Second space 41 First fiber 42 Second fiber 43 Third fiber 43p Flat surface 201, 202 Joint 301 Communication portion

Claims

1. A porous structure made up of a plurality of tubes, comprising: a first tube having a first space therein; and a second tube having a second space therein, wherein the second tube is provided on the first tube so as to intersect with the first tube; and the first tube and the second tube are joined at the intersection where the first tube and the second tube intersect so that the first space and the second space are in communication with each other.

2. The porous structure according to claim 1, wherein the first space and the second space are hollow.

3. A porous structure as described in claim 1 or 2, wherein, at a cross section at the intersection that intersects with the extension direction of the first tube and the extension direction of the second tube, the first tube and the second tube are joined so as to have a constricted portion at the portion where the first space and the second space communicate with each other.

4. A porous structure according to any one of claims 1 to 3, wherein the first tube and the second tube are metal tubes.

5. A porous structure according to any one of claims 1 to 4, wherein the porous structure has a first surface and a second surface spaced apart from each other in the thickness direction, and the surface roughness of the first surface is greater than the surface roughness of the second surface.

6. The porous structure according to claim 5, wherein the plurality of tubes are more densely arranged on the second surface than on the first surface.

7. A porous structure according to claim 5 or 6, wherein the tubes located near the second surface have flat portions that define the second surface.

8. A porous structure according to any one of claims 1 to 7, wherein the porous structure has an arithmetic mean roughness Ra of 5 µm or less, or a maximum height roughness Rz of 20 µm or less.

9. A porous structure according to any one of claims 1 to 8, wherein a first fiber is disposed in the first space, and a second fiber is disposed in the second space.

10. The porous structure according to claim 9, wherein the first fibers are filled in the first spaces, the second fibers are filled in the second spaces, and the first fibers and the second fibers are joined at the intersections.

11. A porous structure according to any one of claims 1 to 10, wherein the outer surfaces of the first tube and the second tube are smooth or substantially smooth.

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