Hierarchical porous metal

Abstract

To provide newly a hierarchical porous metal.SOLUTION: A hierarchical porous metal is provided.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to hierarchical porous metals. [Background technology]

[0002] Non-Patent Document 1 discloses free-standing hierarchical nanoporous Cu materials prepared by selective laser melting 3D printing and a one-step dealloying process as electrocatalysts for methanol oxidation. Non-Patent Document 1 discloses hierarchical structures with macroscale and nanoscale pores.

[0003] Non-Patent Document 2 describes the properties of as-cast hypereutectic Al 75 Cu 25 We present an annealing and electrochemical dealloying approach to fabricate bulk bimodal porous Cu materials from (at.%) precursor alloys. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Zhang, Y.; Sun, X.; Nomura, N.; Fujita, T. Hierarchical Nanoporous Copper Architectures via 3D Printing Technique for Highly Efficient Catalysts. Small 2019, 15, e1805432. [Non-patent document 2] T. Song, M. Yan, Z. Shi, A. Atrens, M. Qian, Creation of bimodal porous copper materials by an annealing electrochemical dealloying approach, Electrochimica Acta, Volume 164, 2015, Pages 288-296, ISSN 0013-4686, https: / / doi.org / 10.1016 / j.electacta.2015.02.217. Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to provide a novel hierarchically porous metal. [Means for solving the problem]

[0006] The present inventors have conducted extensive research to achieve the above object.

[0007] The present invention encompasses the following hierarchically porous metals, in particular hierarchically porous Cu obtained by annealing and selective etching of L-PBF shaped Cu—Al alloys exhibiting an ultrafine eutectic structure. Section 1. A porous metal, It consists of a eutectic alloy, having millimeter-scale pores, micrometer-scale pores, and nanometer-scale pores; A porous metal with a hierarchical structure.

[0008] Section 2. Item 2. The porous metal according to item 1, wherein the micrometer-scale pores have a spherical or pipe-like structure.

[0009] Section 3. A method for producing porous metal, comprising: (1) A process for producing an additive manufacturing body of a eutectic alloy using a laser powder bed fusion (L-PBF) method; (2) annealing the layered manufactured body obtained in step (1); and (3) etching the annealed layered manufactured body obtained in step (2), The porous metal has millimeter-scale pores, micrometer-scale pores, and nanometer-scale pores; A method for producing porous metals with a hierarchical structure.

[0010] Section 4. The L-PBF method is a method in which the cooling rate of the molten metal is 10 5 K·s~10 7 K·s -1 4. The method for producing a porous metal according to Item 3, wherein

[0011] Section 5. 4. The method for producing a porous metal according to Item 3, wherein the layered product has a lattice structure.

[0012] Section 6. 4. The method for producing a porous metal according to Item 3, wherein the annealing is carried out at a temperature 5 K to 50 K lower than the eutectic temperature of the eutectic alloy.

[0013] Section 7. 4. The method for producing a porous metal according to Item 3, wherein the etching is chemical etching and / or electrochemical etching.

[0014] The present invention can provide porous metals with hierarchical structures on the mm (millimeter), μm (micrometer), and nm (nanometer) scales. Nanoporous metals are a type of functional material with unique bicontinuous open porous structure characteristics, making them ideal candidates for various catalytic applications and electrode materials. Hierarchically porous copper (e.g., Cu) materials with millimeter-, micro-, and nano-sized pores are desirable for various applications. [Effects of the Invention]

[0015] The present invention can provide a novel hierarchically porous metal. [Brief explanation of the drawings]

[0016] [Figure 1] Figure 1 shows the phase diagram of a Cu-Al binary alloy created based on multi-component phase diagram calculation software using a commercially available thermodynamic database. [Figure 2] Figure 2 shows the cross-sectional observation results of (a) Cu-70 mol% Al alloy (melted and solidified material) and (b) Cu-80 mol% Al alloy (melted and solidified material). [Figure 3] FIG. 3 shows the observation results of the permanent mold cast material. [Figure 4] Figure 4 shows SEM images of (a) the molten and solidified material and (b) the metal mold cast material after annealing. [Figure 5] FIG. 5 shows a secondary electron image of the alloy powder observed with a scanning electron microscope. [Figure 6] Figure 6 shows a cubic sample that was fabricated using a metal powder laser additive manufacturing device. [Figure 7] Figure 7 shows the relative densities of 25 additive manufacturing bodies measured by the Archimedes method in a bubble chart. [Figure 8] Figure 8 shows the relative density of the formed body, calculated using P·v-1 / 2, a parameter based on Deposited Energy Density (DED) (Equation (1)), which is an index of energy density. [Figure 9] Figure 9 shows SEM images ((a) and (b)) of the additive manufacturing body No. 3 (P = 158 W, v = 1.0 m·s-1). [Figure 10] Figure 10 shows SEM images ((c), (d)) of the additive manufacturing body No. 23 (P = 158 W, v = 2.5 m·s-1). [Figure 11] Figure 11 shows SEM images ((a) and (b)) of the additive manufacturing product No. 18 (P = 158 W, v = 2.2 m·s-1). [Figure 12] Figure 12 shows SEM images ((c), (d)) of the additive manufacturing product No. 25 (P = 240 W, v = 2.5 m·s-1). [Figure 13] FIG. 13 shows SEM images ((a) and (b)) of layered object No. 5. [Figure 14] Figure 14 shows SEM images of layered manufactured bodies No. 4 ((a), (b)), No. 9 ((e), (f)), and No. 10 ((c), (d)). [Figure 15] FIG. 15 shows the results of SEM observation of additive manufacturing bodies No. 3(a), No. 4(b), No. 18(c), No. 23(d), No. 25(e), and No. 5(f), excluding No. 10. [Figure 16] FIG. 16 shows the results of SEM observation of layered manufactured body No. 5 (annealing conditions: (a) 816 K / 12 h, (b) 816 K / 6 h) and No. 10 (annealing conditions: (c) 816 K / 1 h, (d) 816 K / 0.5 h). [Figure 17] FIG. 17 shows the results of SEM observation of layered manufactured body No. 9 ((a) annealing condition at 763 K) and No. 3 ((b) annealing condition at 673 K, (c) annealing condition at 573 K). [Figure 18] FIG. 18 shows a schematic diagram of the process of forming a nanoporous structure by selective etching of an L-PBF shaped body. [Figure 19] Figure 19 shows the results of chemical etching of the layered shaped body No. 3 ((a) 6 h, (a) 24 h) and No. 23 ((c) 6 h, (d) 24 h) performed in chemical etching of the layered shaped body (i). [Figure 20] 20(a) to (c) show the results of chemical etching of layered object No. 5 ((a) 6 hours, (b) 12 hours, (b) 18 hours) performed by chemical etching of layered object (ii). [Figure 21] FIG. 21 shows the results of chemical etching of the layered shaped body No. 4 ((a) 3h, (b) 5h) and No. 10 ((c) 6h, (d) 10h) performed by chemical etching of the layered shaped body (iii). [Figure 22] FIG. 22 shows the results of chemical etching of the layered object No. 5 ((a) 6 h, (b) 9 h) performed by chemical etching of the layered object (iv). [Figure 23]Fig. 23 shows the results of observing the shaped body No. 5 using an FE-SEM (JSM-7001F manufactured by JEOL Ltd.). [Figure 24] Fig. 24 shows the results of observing the shaped body No. 5 using an FE-SEM (GeminiSEM560 manufactured by ZEISS). [Figure 25] FIG. 25 shows the observation results of a sample that was electrochemically etched by electrochemical etching of an additive manufacturing body (ii). [Figure 26] FIG. 26 shows the results of SEM observation of electrochemical etching performed in electrochemical etching of an additive manufacturing body (vi). [Figure 27] FIG. 27 illustrates the relationship between the process conditions up to the annealing step of the present invention and the microstructure. DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention will be described in detail below.

[0018] In this specification, the term "containing" is a concept that encompasses all of "comprise," "consist essentially of," and "consist only of."

[0019] In this specification, when a numerical range is expressed as "A to B," it means "not less than A and not more than B."

[0020] [1] Manufacturing method of hierarchical porous metal The present invention relates to a hierarchical porous metal (preferably Cu) with nanoscale pores and microscale structures, which is produced by annealing and selectively etching an additive manufacturing body with an ultrafine eutectic structure at the submicron level, produced by the L-PBF method.

[0021] The present invention can obtain a hierarchical porous structure by annealing and selectively etching a Cu-70 mol% Al alloy, preferably fabricated by metal additive manufacturing. The formation of nanoporous Cu by selectively etching a binary alloy containing Cu is preferably performed using a combination of Cu-based binary eutectic alloys such as Cu-Al, Cu-Mg, and Cu-Sn.

[0022] By adding nanoscale structures such as unevenness and pores to metal surfaces, functions not found in smooth surfaces, such as catalytic performance and superhydrophilicity, are realized due to an increase in surface area, a high curvature of the surface, and modification of the surface itself. In addition, by adding interconnected pores to the metal surface that act as material transport pathways, it is expected that the metal surface will be able to achieve high efficiency and utilize the internal space.

[0023] It is known that selective etching of binary alloys removes one of the less noble elements (easily ionized and with a high ionization tendency), and the other element (noble metal, a metal that is resistant to oxidation and corrosion and has a low ionization tendency) forms a nanoscale structure, often on the metal surface.

[0024] This invention produces nano- / micro- / milli-scale hierarchical porous metals by adding micro-scale interconnected pores and nano-scale surface structures to a three-dimensional lattice structure with a fine eutectic structure fabricated by the laser powder bed fusion (L-PBF) method.

[0025] The method for producing porous metal of the present invention is characterized by forming spherical or pipe-shaped μm-scale pores by combining structure control by annealing and ultra-rapid solidification by the L-PBF method. 7 Due to the high temperature (approximately K / s), a very fine lamellar (layered) structure (eutectic structure) of α-Al (an Al-rich phase containing a small amount of Cu) and CuAl2 (a Cu-rich phase) is formed. The layer spacing is approximately 0.5 μm. At this stage, before selective etching, no pores have formed.

[0026] Annealing this material transforms the α-Al into spherical and rod-like shapes. The annealing conditions are preferably 816K (816K, 5K lower than the eutectic temperature of Cu-70mol%Al alloy: 821K) for 6 hours. Selective etching of this material removes the α-Al portion, creating spherical and pipe-like μm-scale pores.

[0027] Preparation procedure Hierarchical porous Cu is fabricated from Cu-Al alloys by a combination of laser-assisted powder bed fusion (L-PBF), annealing, and selective etching.

[0028] The method for producing porous metal of the present invention comprises: (1) A process for producing an additive manufacturing body of a eutectic alloy using a laser powder bed fusion (L-PBF) method; (2) annealing the layered manufactured body obtained in step (1); and (3) The method includes a step of etching the annealed layered product obtained in step (2).

[0029] The porous metal of the present invention has millimeter-scale pores, micrometer-scale pores, and nanometer-scale pores and is composed of a hierarchical structure.

[0030] (1) L-PBF modeling The method for producing a porous metal of the present invention includes the step of (1) producing an additively shaped body of a eutectic alloy using a laser powder bed fusion (L-PBF) method.

[0031] The L-PBF (Laser Powder Bed Fusion) method is a type of metal 3D printer. In the L-PBF method, a laser is irradiated onto a thin layer of powder on the bed, solidifying the irradiated area, and then this process is repeated layer by layer to create complex three-dimensional structures. The L-PBF method is characterized by rapid solidification.

[0032] (eutectic alloy) In the method for producing porous metal of the present invention, the following alloys are preferred as eutectic alloys suitable for dealloying methods, from the viewpoint that they can be used to produce porous metals that are made of eutectic alloys, have millimeter-scale pores, micrometer-scale pores, and nanometer-scale pores, and have a hierarchical structure.

[0033] The Cu-Al alloy is preferably Cu-66 to 97 mol % Al, more preferably Cu-70 to 90 mol % Al, and even more preferably Cu-72 to 75 mol % Al.

[0034] The Cu—Sn alloy is preferably Cu-45 to 98 mol % Sn, more preferably Cu-45 to 60 mol % Sn, and even more preferably Cu-45 to 50 mol % Sn.

[0035] The Cu-Mg alloy is preferably Cu-66 to 90 mol % Mg, more preferably Cu-70 to 80 mol % Mg, and even more preferably Cu-70 to 75 mol % Mg.

[0036] The Ni-Al alloy is preferably Ni-75 to 97 mol % Al, more preferably Ni-75 to 85 mol % Al, and even more preferably Ni-78 to 85 mol % Al.

[0037] The Al-Mg alloy is preferably Al-62 to 88 mol % Mg, more preferably Al-65 to 75 mol % Mg, and even more preferably Al-67 to 75 mol % Mg.

[0038] In either case, the second element (Sn in the case of Cu-45.5 to 98.7 mol % Sn) is removed from the eutectic alloy by selective etching, leaving the first element.

[0039] In the method for producing porous metal of the present invention, by using the above-mentioned eutectic alloy, for example, as a preferred embodiment of a Cu-Al alloy, when an L-PBF shaped body is produced, it has a fine lamellar spacing of 1 μm or less, and the CuAl2 phase is also refined to about 1 μm or less, and as a result, a uniform microstructure is formed by annealing.

[0040] On the other hand, when coarse CuAl2 (several tens of micrometers) is present, as in cast materials, the morphology of that part does not change much even after annealing (compare Figure 2(a) with Figure 4(a)). As a result, block-shaped CuAl2 remains, and since the α-Al phase is not visible within it, the structure is not uniform (there are areas with and without α-Al).

[0041] In the porous metal manufacturing method of the present invention, the additive manufacturing object preferably has a lattice structure, from the viewpoint that it is possible to produce a porous metal consisting of a eutectic alloy, having millimeter-scale pores, micrometer-scale pores, and nanometer-scale pores, and having a hierarchical structure. For example, a lattice structure (grid structure) of a Cu-Al alloy is manufactured using L-PBF. The lattice structure is, for example, a lattice structure with a side length of 30 mm.

[0042] [ka]

[0043] This invention uses the L-PBF method, a metal additive manufacturing technology, to manufacture a Cu-70mol%Al alloy. The L-PBF method is the most commonly used metal additive manufacturing technology, and is a method for manufacturing three-dimensional structures layer by layer by repeatedly laying down metal powder and melting and solidifying it using laser irradiation.

[0044] (Cooling rate of molten metal) In the method for producing a porous metal of the present invention, the L-PBF method is preferably used when the cooling rate of the molten metal is 10 or less, from the viewpoint that the L-PBF method can satisfactorily produce a porous metal made of a eutectic alloy, having millimeter-scale pores, micrometer-scale pores, and nanometer-scale pores, and having a hierarchical structure. 5 K·s~10 7 K·s -1 is.

[0045] In the L-PBF method, the cooling rate of the molten metal is preferably 10 3 K·s or more, more preferably 10 5 K·s~10 7 K·s -1 is.

[0046] In the method for producing porous metal of the present invention, the L-PBF method is used, and the cooling rate of the molten metal is 10 5 K·s -1 ~10 7 K·s -1 This is extremely fast, and rapid cooling allows for the creation of a fine structure. By using the L-PBF method, a scale-like structure called a molten pool structure is formed. By using metal additive manufacturing, there is a high degree of freedom in the shape of the sample to be manufactured. By using metal additive manufacturing, it is possible to create three-dimensional structures according to the purpose.

[0047] (2) Annealing The method for producing a porous metal of the present invention includes (2) a step of annealing the layered shaped body obtained in step (1).

[0048] Annealing is a heat treatment that is performed on materials produced by casting, etc., to give them toughness (some flexibility). Materials produced by casting are often hard and brittle, and in such cases, annealing is performed.

[0049] The lattice structure formed by L-PBF is then annealed (heat treated). This refers to heat treatment to change the structure of the material. The microstructure (structure observed under a microscope) of the lattice structure changes. In particular, by using a eutectic alloy to create a lattice structure using the L-PBF method, a fine lamellar (layered) eutectic structure can be obtained, making the structure more susceptible to change by annealing.

[0050] In the porous metal manufacturing method of the present invention, annealing of the L-PBF shaped body is preferably carried out at a temperature 5K to 50K lower than the eutectic temperature of the eutectic alloy for a time period ranging from 0.5 hours to 24 hours, in order to effectively produce a porous metal made of a eutectic alloy, having millimeter-scale pores, micrometer-scale pores, and nanometer-scale pores and a hierarchical structure.

[0051] The L-PBF shaped body is annealed preferably at a temperature 1 K to 300 K lower than the eutectic temperature of the eutectic alloy, more preferably at a temperature 5 K to 50 K lower than the eutectic temperature of the eutectic alloy, and even more preferably at a temperature 1 K to 10 K lower than the eutectic temperature of the eutectic alloy.

[0052] The L-PBF shaped body is annealed for a time period ranging from 0.1 to 100 hours, more preferably from 0.5 to 24 hours, and even more preferably from 0.5 to 2 hours.

[0053] For example, the eutectic temperature of a Cu-70 mol% Al alloy is 821 K. An L-PBF shaped body of a Cu-70 mol% Al eutectic alloy is preferably annealed at 816 K to 771 K, which is 5 K lower than the eutectic temperature, for a time period ranging from 0.5 to 24 hours.

[0054] α-Al is not pure Al, but Al containing other elements. In the examples, α-Al contains Cu. Annealing the solidified structure causes the structure to become coarse.

[0055] The lamellar eutectic structure of the α-Al phase and CuAl2 phase present in the Cu-70mol%Al alloy prepared in this invention has a large interfacial area, and annealing causes coarsening to reduce the interfacial energy. As a result, the layered eutectic structure separates, and the α-Al phase and CuAl2 phase aggregate. Furthermore, due to the rapid diffusion of Cu atoms in the α-Al phase, the amount of Cu in the α-Al phase decreases, making it easier for Al atoms to be eluted by selective etching.

[0056] In the porous metal manufacturing method of the present invention, by setting the annealing conditions as described above, as a preferred mode of annealing, for example, the L-PBF shaped body becomes a continuous phase of lamellar-disconnected CuAl2 phase, and α-Al is dispersed in the CuAl2 phase in pipe-shaped or spherical shapes on a micrometer scale. As a result, even if the α-Al phase is removed by chemical or electrochemical etching, the material maintains sufficient strength.

[0057] In this invention, a microscale structure is imparted to nanoporous Cu. To form the microscale structure, the extremely fine α-Al / CuAl2 eutectic structure of the additively manufactured body is annealed to control the morphology of the α-Al phase and make the CuAl2 phase continuous.

[0058] (3) Selective etching The method for producing a porous metal of the present invention includes (3) a step of etching the annealed layered shaped body obtained in step (2).

[0059] Nanoporous metals, which have nanoscale pores, have properties not found in bulk metals, such as a large specific surface area and fluid permeability, and are used in applications such as catalysts and electrodes. Nanoporous metals are preferably fabricated by selective etching, template methods, etc.

[0060] Selective etching methods are also called dealloying or dealloying corrosion, and are broadly divided into chemical and electrochemical etching methods that utilize the difference in standard electrode potential, molten metal methods that utilize the difference in affinity between metals, and methods that use molten salts.

[0061] In the method for producing a porous metal of the present invention, the etching is preferably chemical etching and / or electrochemical etching, from the viewpoint that it is possible to produce a porous metal that is composed of a eutectic alloy, has millimeter-scale pores, micrometer-scale pores, and nanometer-scale pores, and has a hierarchical structure.

[0062] The method of the present invention, which utilizes the difference in standard electrode potential, decomposes a binary alloy using chemical or electrochemical techniques to remove only the less noble elements, thereby forming nanoscale pores and producing nanoporous metals.

[0063] Selective etching is a method of chemically removing only one element (A or B) from an AB alloy (dissolving the metal in acid or alkali). It is also called dealloying. In selective etching, only the less noble metal (high ionization tendency or low standard electrode potential) is dissolved in solution, leaving only the other noble element, creating a nanoscale porous structure.

[0064] When annealed Cu-Al alloys are immersed in acid, the Cu does not dissolve, but only the Al dissolves, resulting in the formation of μm-scale pores. Furthermore, Al dissolves from CuAl2, resulting in the formation of nm-scale pores.

[0065] In the present invention, chemical etching using HCl and electrochemical etching using a three-electrode cell in which a current is passed through the Cu-Al alloy as the working electrode are preferably employed. In either case, only the metal with a lower standard electrode potential (which is an intrinsic value of the metal) is selectively removed (this applies to Al in the case of a Cu-Al alloy).

[0066] Chemical etching Chemical etching is effective in producing porous metal made of a eutectic alloy, having millimeter-scale pores, micrometer-scale pores, and nanometer-scale pores, and having a hierarchical structure. Therefore, the annealed additive manufacturing body is preferably immersed in 0.25M to 1M HCl for approximately 3 to 24 hours.

[0067] For chemical etching, the annealed layered product is preferably immersed in 0.01M to 10M HCl, 0.01M to 10M H2SO4, 0.01M to 10M HNO3, or the like, more preferably 0.25M to 1M HCl, 0.25M to 1M H2SO4, or the like, and even more preferably immersed in 0.25M to 0.5M HCl.

[0068] Chemical etching is a simple method in which a sample is immersed in HCl for a certain period of time, and is the most suitable method for forming a nanoporous structure.

[0069] In the porous metal manufacturing method of the present invention, by setting the above-mentioned chemical etching conditions, as a preferred embodiment of chemical etching, for example, the α-Al phase of the annealed L-PBF shaped body is completely removed, resulting in the creation of micro-scale pores, and also the removal of Al from the surface of the CuAl2 phase, resulting in the creation of nano-scale pores or surface irregularities.

[0070] Electrochemical etching Electrochemical etching is preferably performed using 1M NaCl and 1M CuSO4 as an electrolyte, and at a current of 1 mA cm for the additive manufacturing body, from the viewpoint that it can effectively produce a porous metal made of a eutectic alloy, having millimeter-scale pores, micrometer-scale pores, and nanometer-scale pores, and having a hierarchical structure. -2 ~20mA·cm -2 A current is passed at a current density of about 1 to 5 hours.

[0071] For electrochemical etching, a current is passed through the annealed layered product using an electrolyte of preferably 1M NaCl (e.g., 0.5M to 5M), 1M CuSO4 (e.g., 0.5M to 5M), or 1M KCl (e.g., 0.5M to 5M), more preferably 1M NaCl, 1M CuSO4, or 1M KCl.

[0072] The electrochemical etching is preferably performed on the annealed additive manufacturing body at a rate of 1.0×10 -5 mA / cm 2 ~200mA / cm 2 A current density of about 1 mA / cm is preferred. 2 ~100mA / cm 2 A current density of about 10 mA / cm is more preferable. 2 ~50mA / cm 2 A current is passed at a current density of about 1000 .mu.m.

[0073] In the electrochemical etching, a current is passed through the annealed layered product for preferably about 0.5 to 48 hours, more preferably about 1 to 24 hours, and even more preferably about 3 to 6 hours.

[0074] Electrochemical etching allows the selection of the type and amount of metal removed by adjusting the current density and potential. By using an appropriate electrolyte, the removed metal can be recovered at the counter electrode, making it possible to recycle resources.

[0075] In the porous metal manufacturing method of the present invention, by setting the above-mentioned electrochemical etching conditions, as a preferred embodiment of electrochemical etching, for example, the α-Al phase of the annealed L-PBF shaped body is completely removed, resulting in the formation of micro-scale pores, and also the removal of Al from the surface of the CuAl2 phase, resulting in the formation of nano-scale pores or surface irregularities.

[0076] [2] Hierarchical porous metal The porous metal of the present invention is made of a eutectic alloy, has millimeter-scale pores, micrometer-scale pores, and nanometer-scale pores, and has a hierarchical structure.

[0077] In the porous metal of the present invention, the morphology of the micrometer-scale pores is preferably a spherical or pipe-like (preferably elongated pipe-like) structure.

[0078] The porous metal of the present invention is a porous metal (hierarchical porous metal) having hierarchical structures on the mm (millimeter), μm (micrometer), and nm (nanometer) scales. The porous metal of the present invention is a metal material having pores on the mm, μm, and nm scales, and the metal is preferably a eutectic alloy, more preferably a Cu alloy.

[0079] (Fabrication of Cu-Al alloys by additive manufacturing) The present invention relates to the [1] Manufacturing method of hierarchical porous metal As explained in the previous section, we will realize a structure with pores at three scales (hierarchical structure of hierarchical porous metal) by additive manufacturing, annealing, and selective etching. We will impart microscale pores to nanoporous Cu by annealing and selective etching. We will develop hierarchical porous Cu by imparting nano / microscale pores to a millimeter-scale lattice structure.

[0080] Lattice structure (Chemical Formula 1): millimeter (mm)-scale pores. In the hierarchical porous metal of the present invention, the millimeter (mm)-scale pores are preferably pores of about 0.2 mm to 15 mm, more preferably pores of about 1 mm to 5 mm, and even more preferably pores of about 2 mm to 3 mm. The mm-scale pores of the hierarchical porous metal contribute to the material transport (gas) of the hierarchical porous metal. The hierarchical porous metal has increased fluid permeability and heat exchange space.

[0081] The α-Al phase is formed into micro (μm)-scale pores by selective etching. In the hierarchical porous metal of the present invention, the micro (μm)-scale pores are preferably pores of about 1 μm to 300 μm, more preferably pores of about 5 μm to 100 μm, and even more preferably pores of about 20 μm to 50 μm. The μm-scale pores of the hierarchical porous metal contribute to fluid permeation through the hierarchical porous metal. The surface area of ​​the hierarchical porous metal is increased. They serve as material transport pathways for reactants and products, increasing the efficiency of reactions.

[0082] Cu surface: nano (nm)-scale pores. In the hierarchical porous metal of the present invention, the nano (nm)-scale pores are preferably pores of about 100 nm or less, more preferably pores of about 10 nm or less, and even more preferably pores of about 0.1 nm to 1 nm. The nm-scale pores of the hierarchical porous metal contribute to the creation of functions of the hierarchical porous metal. Hierarchical porous metals lead to large surface areas, large curvatures, and reaction promotion. Nanoporous metals. They have properties not found in bulk metals, such as a high specific surface area and fluid permeability. The hierarchical porous metal of the present invention, due to its nano (nm)-scale pores, is suitable for applications such as heat exchange, batteries, and catalysts.

[0083] The porous metal of the present invention is particularly preferably one in which the μm-scale pores have a spherical or elongated pipe-like structure. The porous metal of the present invention has high connectivity of the solid portion, high strength, and the μm-scale pores connected in a pipe-like manner impart fluid permeability to the interior of the material, has a large surface area, and exhibits capillary force.

[0084] The porous metal of the present invention has a hierarchical porous structure formed by combining nanoscale and microscale structures, and having structures of different scales maximizes the properties of the nanoporous metal.

[0085] Specifically, while the nanoporous structure acts as a reaction site, the microscale structure acts as a transport route for reactants and products, facilitating the supply of materials necessary for the reaction. In addition, by making the surface of a three-dimensional structure such as a lattice nanoporous, it becomes possible to utilize the nanoporous structure formed only on the surface of the material in three dimensions.

[0086] One way to utilize nanoporous structures is to promote boiling heat transfer. Boiling heat transfer is widely used in air conditioning equipment and heat exchangers in both industrial and everyday life. The principle is that when a liquid boils, it absorbs energy (heat) from its surroundings, but instead undergoes a phase transformation. Because it can absorb heat from its surroundings through a phase transformation, a phenomenon that does not generate any new harmful substances, it is used in a variety of devices as a cooling device that does not have a negative impact on the environment.

[0087] Boiling heat transfer can be enhanced by micro- and nano-scale surface modification. Porosity of the surface provides high surface area, nucleation sites for boiling nuclei, wettability, and wicking, improving the critical heat flux. Furthermore, micro- and nano-biporous copper surfaces have the advantage of superior heat transfer coefficients and shorter bubble growth times during boiling compared to bulk copper with a flat surface.

[0088] In the field of electrochemistry, nanoporous structures are used in lithium-ion batteries. Lithium-ion batteries are widely used in electrical appliances and electric vehicles, and improvements in their safety and rapid charge / discharge characteristics are desirable. The increased surface area achieved by adding ordered nanoporous structures to metal surfaces is required for high-surface-area conductive current collectors and catalytic materials. For example, nanoporous gold can act as a catalyst for the decomposition of methyl orange.

[0089] The increase in surface area and surface modification due to the nanoporous structure can improve the performance of the metal itself and bring out performance that cannot be seen in the bulk state. [Example]

[0090] Hereinafter, the embodiments of the present invention will be described more specifically based on examples. However, the present invention is not limited to the scope of the examples.

[0091] [1] Fabrication of hierarchical porous metals Specifically, nano- / micro-scale pores were created in Cu by combining selective etching with the morphological change caused by annealing to the α-Al / CuAl2 eutectic structure of a Cu-70mol%Al alloy. In the examples, we show the morphological change of the α-Al / CuAl2 eutectic structure by annealing and the observation results of the nano- / micro-scale structure after selective etching, with the aim of controlling the morphology of the micro-scale pores formed by selective etching.

[0092] (1) Microstructure of Cu-Al alloys with different solidification conditions There is a very strong relationship between the structure of a metal and the cooling rate from the molten state. If the cooling is done slowly, the structure will become coarse, but as the cooling rate increases, the solidification of the molten metal will occur in a shorter time, making it difficult for atoms to diffuse, resulting in a finer structure. The cooling rate of molten solidified material is 10 -1 K·s -1 and for permanent mold casting materials, it is 10 2 K·s -1 is.

[0093] The layered product of the present invention (cooling rate is 10 5 K·s -1 ~10 7 K·s -1 As comparative materials for the above, melt-solidified material and metal mold cast material were prepared using a high-frequency melting furnace.

[0094] Experimental Method Alloy composition of Cu-Al molten solidified material and metal mold casting material Figure 1 shows the phase diagram of a Cu-Al binary alloy created using the commercially available thermodynamic database PanAluminum and the multi-component phase diagram calculation software Pandat (manufactured by CompuTherm LLC). According to Figure 1, the eutectic composition of the Cu-Al alloy is Al-17 mol% Cu, and the eutectic temperature is 821 K. Furthermore, in the range of Cu from 17 mol% to 33 mol%, it has a primary Θ-CuAl2 phase and a lamellar eutectic structure consisting of the Θ-CuAl2 phase and the α-Al phase. The composition of the CuAl2 phase is Al-33 mol% Cu.

[0095] The melt-solidified materials were prepared with two compositions: Cu-70mol%Al and Cu-80mol%Al.The permanent mold cast material was also prepared with the composition Cu-70mol%Al.

[0096] The raw powders used for the molten solidified material were Cu powder (purity 99.9%, particle size 45 μm, manufactured by Kojundo Chemical Laboratory) and Al powder (purity 99.99%, particle size 45 μm, manufactured by Kojundo Chemical Laboratory). The Cu powder and Al powder were weighed so that the Cu:Al molar ratio was 3:7 or 2:8, and then dry mixed for 10 minutes and cold pressed to compact the powder. The metal mold casting material used was Cu-70 mol % Al powder (manufactured by Toyo Aluminum).

[0097] Melted and solidified material manufacturing method The mixed powder of Cu and Al was placed in an alumina Tammann tube (Nikkato SSA-H Tammann T2) and heated to 1373 K in an Ar atmosphere in a high-frequency melting furnace (Daia Vacuum Co., Ltd.), and held at that temperature for 0.5 h. After that, heating was stopped and the furnace was cooled (furnace-cool, cooling rate: 10 -1 K·s -1 ) and the sample was removed when the temperature had dropped to room temperature.

[0098] Method for producing metal mold casting materials First, Cu-70 mol% Al alloy powder was placed directly into an alumina Tammann tube (Nikkato SSA-H Tammann T2) and heated to 1373 K in an induction furnace under an Ar atmosphere. However, when the molten metal was poured into the Tammann tube, the metal adhered to the tube and did not flow out. Therefore, a molten solidified material was prepared using Cu-70 mol% Al alloy powder (Toyo Aluminum) in the same manner as in the molten solidified material preparation method. The material was placed in a magnesium oxide crucible (Nikkato MG special refractory crucible), heated in an induction furnace under an Ar atmosphere, and poured into a mold when the alloy powder was melted (casting, cooling rate: 10 2 K·s -1 After that, the heating was stopped and the sample was removed when the temperature had dropped to room temperature.

[0099] Annealing of melt-solidified materials and metal mold cast materials To investigate the morphological changes of the θ-phase and α-Al phase with respect to temperature, the melt-solidified material and the metal mold cast material were annealed. An electric furnace (manufactured by Isekyu Co., Ltd.) was used for annealing. The eutectic temperature of Cu-70mol%Al alloy is 821K. Therefore, annealing was carried out for 24 hours at 816K, which is 5K lower than the eutectic temperature. After 24 hours at 816K, the power to the furnace was turned off, and the sample was removed after the temperature inside the furnace had returned to room temperature.

[0100] Microstructural observation of melt-solidified material and metal mold cast material before and after annealing Cross-sectional microstructures of the molten and solidified materials and the permanent-mold cast materials were observed before and after annealing. The prepared samples were cut into the XY plane perpendicular to the preparation direction using an isomet (Buehler), and this cross section was used as the observation surface. After embedding in resin so that the cut surface served as the observation surface, the samples were wet-polished with SiC emery paper (Marumoto Struers Co., Ltd., #320, #500, #800, #1200, #2400) and then mirror-finished with diamond slurry (3 μm, 1 μm) and colloidal silica (Musashino Electronics Chemical Liquid 0.05 μm, pH 9.8). A scanning electron microscope (SEM: JEOL JSM-IT500) was used for observation.

[0101] Results and Discussion Observation results of molten and solidified material Figure 2(a) shows the cross-sectional observation results of a Cu-70mol%Al alloy (melted and solidified material). The primary Θ-CuAl2 phase exhibited a rhombic morphology with sides of approximately 200μm, and a lamellar eutectic structure of α-Al and Θ phases was formed between the primary Θ phases. The spacing of the lamellar eutectic structures was approximately 2μm at the finest point, but at positions away from the primary phase, the eutectic structures exhibited a nearly spherical shape with a diameter of approximately 10μm. The volume ratio of the α-Al phase to the Θ phase in the phase diagram of Cu-70mol%Al alloy is 1:10, and it was confirmed that this was also the case for the sample prepared in this study.

[0102] Figure 2(b) shows the cross-sectional observation results of a Cu-80mol%Al alloy (melted and solidified material). The primary Θ-CuAl2 phase exhibited a circular shape with a diameter of approximately 10 μm, around which a lamellar eutectic structure consisting of the α-Al phase and the Θ phase was formed. The spacing between the lamellar eutectic structures was approximately 3 μm. The volume ratio of the α-Al phase to the Θ phase in the phase diagram of Cu-80mol%Al alloy is 2:3, and it was confirmed that this was also the case in the sample prepared in this study.

[0103] Observation results of permanent mold casting material Figure 3 shows the observation results of the permanent mold cast material. The primary Θ-CuAl2 phase exhibited a block-like morphology with sides of approximately 20 μm, and a lamellar eutectic structure of the α-Al phase and the Θ phase was formed between the primary Θ phases. The spacing of the lamellar eutectic structure was approximately 0.5 μm. Compared to the melt-solidified material, the side length of the primary crystals was approximately 0.1 times longer, and the lamellar spacing was approximately 0.25 times larger. This is thought to be because the cooling rate of the permanent mold cast material was approximately 1,000 times faster than that of the melt-solidified material, resulting in a finer structure in the permanent mold cast material.

[0104] Structural changes due to annealing of molten and solidified materials Figure 4(a) shows an SEM image of the molten-solidified material after annealing. When the molten-solidified material was annealed, the relatively fine lamellar eutectic structure formed around the primary crystals became aggregated with the α-Al phase and the Θ phase, respectively, and the fine lamellar structure disappeared. However, the relatively coarse spherical structure present away from the primary crystals showed little change from its pre-annealing state. This suggests that the morphological changes in the structure due to annealing are driven by the interfacial energy present at the interface between the α-Al phase and the Θ phase, and that changes are more likely to occur in fine lamellar structures. Furthermore, since the size of the aggregated α-Al phase and the Θ phase were both approximately 10 μm, it is thought that little change was observed in structures that were larger than 10 μm before annealing.

[0105] Structural changes due to annealing of permanent mold cast material Figure 4(b) shows an SEM image of the permanent-molden-die cast material after annealing. The fine lamellar structure present between the block-like primary crystals before annealing is no longer visible, and only the α-Al phase remains between the primary crystals. This is thought to be due to the decomposition of the lamellar eutectic structure consisting of the α-Al and Θ phases present between the primary crystals, resulting in the coarsening of the α-Al phase and the absorption of the eutectic Θ phase into the primary crystal. In fact, the primary crystal Θ phase was block-shaped with sides of approximately 20 μm before annealing, but after annealing, its sides grew to approximately 50 μm. Furthermore, the morphological changes in the primary crystals and eutectic structure were greater than in the melt-solidified material. This is thought to be because the lamellar spacing of the permanent-molden-die cast material before annealing was approximately 0.25 times that of the melt-solidified material, making it easier for changes in the microstructure to occur, driven by interfacial energy.

[0106] Summary We produced melt-solidified materials and metal mold cast materials and investigated the changes in the structure due to annealing. By producing melt-solidified materials and metal mold cast materials and comparing the initial structure with the structure after annealing, we found that the fineness of the initial structure, which is caused by differences in cooling rate, has a significant effect on the changes in the structure due to annealing.

[0107] Furthermore, it was found that annealing a lamellar eutectic structure consisting of the α-Al phase and the Θ phase causes each phase to aggregate and coarsen, and the eutectic Θ phase is absorbed into the primary Θ phase, resulting in a connected Θ phase.

[0108] The cooling rate from the molten state of the molten solidified material is 10 -1 K·s -1 The cooling rate of the metal mold casting material is about 10 2 K·s -1 Observation using an SEM confirmed that the eutectic structure consisting of the α-Al phase and CuAl2 phase, and the structural morphology of the primary crystal CuAl2 phase after solidification, differ between the melt-solidified material and the metal mold cast material. This is due to the difference in cooling rate between the two. In the melt-solidified material, the primary crystal CuAl2 phase exhibited a rhomboidal shape with sides of approximately 200 μm, and a eutectic structure with a width of approximately 2 μm was formed between the primary crystals. In the metal mold cast material, the primary crystal CuAl2 phase exhibited a block-like shape with sides of approximately 20 μm, and the spacing of the eutectic structure was approximately 0.5 μm.

[0109] When both materials were annealed, coarsening of the α-Al and CuAl2 phases was observed. In the fine eutectic structure of the melt-solidified material, the α-Al and CuAl2 phases each aggregated, but little change was observed in the eutectic structure, which was originally about 10 μm in size. In addition, significant coarsening of the CuAl2 phase and aggregation of the α-Al phase were observed in the permanent mold cast material. This is thought to be because the spacing of the eutectic structure of the permanent mold cast material was smaller than that of the melt-solidified material, making it easier for structural changes driven by interfacial energy to occur.

[0110] From the above, it was found that increasing the cooling rate of the molten metal makes the initial structure finer, and the effects of annealing are more likely to be seen.

[0111] (2) Fabrication of hierarchical porous metals (2-1) L-PBF additive manufacturing of Cu-Al alloy and changes in microstructure due to annealing First, Cu-70mol%Al alloy pieces were prepared by melt solidification, die casting and L-PBF method.

[0112] In the L-PBF method, the laser power (P) was set to 79 W to 240 W, and the scanning speed (v) was set to 1.0 m s-1 ~2.5m·s -1 The shaped bodies were fabricated under 25 conditions, with the hatch distance (S) set to 100 μm. The relative density of the shaped bodies was then measured using the Archimedes method.

[0113] L-PBF additive manufacturing of Cu-Al alloy Laser powder bed fusion (L-PBF) is a process that uses a cooling rate of 10 5 K·s -1 ~10 7 K·s -1 One of the features of this method is that it can produce a very fine structure very quickly.

[0114] A Cu-70mol%Al alloy was prepared using the L-PBF method, and the relative density was measured using the Archimedes method. The densely shaped sample was then annealed to form a continuous Θ-CuAl2 phase and control the morphology of the α-Al phase.

[0115] Experimental Method Alloy composition of Cu-Al additive manufacturing body The raw material powder used was a Cu-70 mol % Al powder (manufactured by Toyo Aluminum Co., Ltd.) produced by gas atomization.

[0116] A secondary electron image of the alloy powder observed under a scanning electron microscope is shown in Figure 5. The alloy powder was spherical.

[0117] Table 1 shows the composition of the alloy powder.

[0118] [Table 1]

[0119] Layered object manufacturing method and laser conditions Figure 6 shows a cubic sample manufactured by additive manufacturing using a metal powder laser additive manufacturing system, ProX DMP200, manufactured by 3D Systems. The additive manufacturing system was manufactured using a metal powder laser additive manufacturing system, ProX DMP200, manufactured by 3D Systems. The laser scanning speed (v) was 1.0 m·s-1 ~2.5m·s -1 We attempted to fabricate a 10mm x 10mm x 10mm cubic sample under the following conditions: laser power (P) 79W to 240W, hatch distance 100μm, layered powder thickness 30μm, and spot diameter 100μm.

[0120] Table 2 shows the laser conditions for the 25 samples fabricated this time, along with the numbers of the specimens. From now on, we will refer to the specimens fabricated as No. X or No. X (P = YW, v = Z m s -1 ) is expressed in the form

[0121] [Table 2]

[0122] Relative density measurement of additively manufactured objects using the Archimedes method The relative density of the additively manufactured object was measured using the Archimedes method. With the Archimedes method, the sample is submerged in water, the volume of the sample is calculated from the buoyancy and the density of the water, and the density is calculated by dividing the sample mass by the volume. The relative density is calculated as a percentage of the standard density by dividing the calculated density by the standard density.

[0123] When a Cu-70mol%Al alloy is melted and solidified, primary crystals of the CuAl2 phase and a lamellar eutectic structure of the α-Al / CuAl2 phase are formed. The density of the α-Al phase is 2.70 g cm, the density of pure Al. -3 The density of the CuAl2 phase is 4.36 g cm -3 On the Cu-Al equilibrium diagram, the volume ratio of the CuAl2 phase to the α-Al phase is 5:76. Therefore, the density of the standard density Cu-70mol%Al alloy is 4.2576g cm -3 is.

[0124] Annealing of additive manufacturing bodies The melt-solidified material / metal mold casting material / L-PBF molded body was annealed under several conditions to investigate the change in structure with respect to annealing temperature and time. The annealing temperature was 573K to 816K and the annealing time was 0.5h to 24h.

[0125] The relative density was measured using the Archimedes method, and the pieces that were found to have a high density were then annealed.

[0126] Table 3 shows the sample numbers and annealing conditions.

[0127] [Table 3]

[0128] First, the specimens were annealed at 816 K for 24 hours, the same temperature as the molten and solidified specimens and the metal mold cast specimens. Then, the specimens were annealed at 816 K for 12 hours, 6 hours, 1 hour, and 0.5 hours, at 763 K for 1 hour, at 673 K for 1 hour, and at 573 K for 1 hour. After annealing, the electric furnace was turned off, and the specimens were removed after the temperature inside the furnace had returned to room temperature.

[0129] Microstructure observation of additive manufacturing body before and after annealing Among the additively manufactured bodies, samples that had been manufactured at high density and samples that had been annealed were cut using an isomet (made by Buehler) in the XZ plane parallel to the manufacturing direction, and this cross section was used as the observation surface. The XZ plane of the samples that had been embedded in resin (the same as for microstructural observations of molten solidified materials and metal mold cast materials before and after annealing) and polished was then subjected to microstructural observation using a scanning electron microscope (SEM: JEOL JSM-IT500).

[0130] Results and Discussion Relative density measurement Figure 7 shows the relative density of 25 additively manufactured bodies measured by the Archimedes method in a bubble chart. No. 4 (P = 199 W, v = 1.0 m s -1 ) showed a maximum relative density of 94.7%.

[0131] Figure 8 shows the relative density of the molded body as a function of P·v, a parameter based on Deposited Energy Density (DED) (Equation (1)), which is an index of energy density. -1 / 2 The results are shown in the table below. DED is expressed by the following formula.

number

[0132] A is the laser absorptivity, D is the thermal diffusivity, and σ is the laser spot diameter. When the laser parameters are limited to the laser power P and the laser scanning speed v, the DED is P·v -1 / 2 Although there is another method of using volumetric energy density as an index for organizing the relative density of a sintered body, the use of DED allows for a better organization of the relative density of a sintered body.

[0133] Relative density is P·v -1 / 2 It increases with increasing 1 / 2 m -1 / 2 More than P·v -1 / 2 The relative density of the samples printed under different P and v conditions showed a constant value of almost 94%. -1 / 2 is predicted using

[0134] Microstructure observation of additive manufacturing products We narrowed down the 25 additive manufacturing models to a few conditions and conducted microstructural observations based on the following two themes. (i) Understand the overview of the microstructure of the additive manufacturing object. (ii) Observe the high-density fabricated sample.

[0135] In (i), first, No. 3 (P = 158 W, v = 1.0 m s -1 ) and No. 23 (P = 158 W, v = 2.5 m s -1 ) was observed. Since P = 158 W is the intermediate value of the laser power handled in this study, v was 1.0 m s -1 and 2.5 m·s -1 It was thought that by comparing the structures produced under these conditions, it would be possible to understand the relationship between microstructural morphology and laser parameters.

[0136] Figure 9 shows the results of the additive manufacturing process No. 3 (P = 158 W, v = 1.0 m s -1) are shown as SEM images ((a)(b)). It was found that cracks had occurred in part of the surface. This is thought to be because, during manufacturing using the L-PBF method, the next layer of metal powder is laid so that it overlaps the solidified layer, and as it melts and solidifies by laser irradiation, stress concentrates in one area, causing solidification cracks. Furthermore, observation at high magnification confirmed the molten pool structure that is unique to objects manufactured using the L-PBF method. Furthermore, the molten pool structure was formed by a long, thin structure about 1 μm wide. The bright areas are thought to be the CuAl2 phase, and the dark areas are the α-Al phase.

[0137] Figure 10 shows the results of the additive manufacturing process No. 23 (P = 158 W, v = 2.5 m s -1 ) are shown as SEM images ((c)(d)). It was found that the object fabricated under these conditions had voids throughout the entire sample. This is thought to be because the laser scanning speed was too fast, resulting in insufficient laser energy density and insufficient melting of the alloy powder. The presence of a molten pool structure and the internal structure of the molten pool were similar to those of No. 3.

[0138] Then, No. 18 (P = 158 W, v = 2.2 m s -1 ) and No. 25 (P = 240 W, v = 2.5 m s -1 ) were observed. The former was selected to investigate the effects of microstructural morphology and v. The latter, which has the same v as No. 23 but a different P, was selected to investigate the effects of laser power and microstructural morphology.

[0139] Figure 11 shows the results of the additive manufacturing model No. 18 (P = 158W, v = 2.2m s -1 SEM images ((a)(b)) of sample No. 23 are shown. This sample was found to have voids, just like No. 23. The microstructural morphology was similar to that described above.

[0140] Figure 12 shows the results of the additive manufacturing process No. 25 (P = 240 W, v = 2.5 m s -1 ) are shown in SEM images ((c) and (d)). Although voids and cracks were observed on the surface, the amount of voids was smaller than in No. 23. The microstructural morphology was similar to that described above.

[0141] In (ii), we observed the sample that was fabricated with high density. No. 5 (P = 240 W, v = 1.0 m s -1 , relative density: 94.1%), No. 4 (P = 199W, v = 1.0 m s -1 , relative density: 94.7%), No. 10 (P = 240 W, v = 1.4 m s -1 , relative density: 94.2%), No. 9 (P = 199W, v = 1.4 m s -1 Four types of shaped bodies were observed, each with a relative density of 93.9%.

[0142] Figure 13 shows SEM images ((a)(b)) of additive manufacturing product No. 5. It was found that additive manufacturing product No. 5 had few cracks overall and no voids (Figure 13(a)). Furthermore, when observed at a higher magnification, it was found that even finer structures with a width of approximately 0.15 μm existed between the elongated structures within the molten pool structure (Figure 13(b)). From this, it is thought that the elongated structures with a width of approximately 1 μm (bright areas) are primary crystals of the CuAl2 phase, the dark areas are the α-Al phase, and the structure with a width of approximately 0.15 μm is a eutectic of the CuAl2 phase and the α-Al phase.

[0143] Figure 14 shows SEM images of additively manufactured bodies No. 4 ((a)(b)), No. 10 ((c)(d)), and No. 9 ((e)(f)). No. 4 exhibited the highest relative density of all the bodies fabricated in this study. It was found that the high-density manufactured bodies, including No. 10 and No. 9, had few voids and had similar microstructural morphologies (Figures 14(a)-(f)). Therefore, in future annealing and selective etching experiments, we conducted experiments assuming that the laser conditions had no effect on changes in the microstructural morphology.

[0144] Changes in microstructural morphology due to annealing First, annealing was performed on the shaped bodies No. 3, No. 4, No. 5, No. 10, No. 18, No. 23, and No. 25 at 816 K for 24 hours.

[0145] Figure 15 shows the SEM observation results for the additively manufactured bodies Nos. 3(a), 4(b), 18(c), 23(d), 25(e), and 5(f), excluding No. 10. Annealing at 816 K for 24 hours resulted in the decomposition of the lamellar structure, the formation of a continuous CuAl2 phase, and the α-Al phase changing to a spherical shape with a diameter of several micrometers. No. 5 was observed without polishing to compare the microstructural changes due to annealing on the surface of the manufactured body with those of a polished cross-section under the same conditions. While the surface of an additively manufactured body is generally considered to be oxidized, we were able to confirm that the same morphological changes as those observed in the cross-section also occurred on the surface of the manufactured body.

[0146] Figure 16 shows the SEM observation results for additive manufacturing bodies No. 5 (annealed under the following conditions: (a) 816 K / 12 h, (b) 816 K / 6 h) and No. 10 (annealed under the following conditions: (c) 816 K / 1 h, (d) 816 K / 0.5 h). Since it was thought necessary to reduce the annealing time in order to obtain a more continuous α-Al phase, No. 5 was annealed under the following conditions: 816 K / 12 h, 6 h, and No. 10 was annealed under the following conditions: 816 K / 1 h, 0.5 h. After annealing for 12 h, connected α-Al phases measuring several μm in width and approximately 10 μm in length were observed in places. In the annealing times of 6 h, 1 h, and 0.5 h, the width of the connected α-Al phase tended to decrease as the annealing time decreased, but many completely spheroidized α-Al phases were observed, and the α-Al phase did not become a continuous phase (Figures 16(a) to (d)).

[0147] Figure 17 shows the SEM observation results for additive manufacturing bodies No. 9 (annealed at (a) 763 K) and No. 3 (annealed at (b) 673 K, and (c) 573 K). The annealing time was fixed at 1 h, and the annealing temperature was varied. No. 9 was annealed at 763 K, and No. 3 at 673 K and 573 K. Annealing at 763 K revealed more connected α-Al phases than annealing at 816 K (Figure 17(a)). Annealing at 673 K revealed that the α-Al phase exhibited anisotropy toward the center of the molten pool structure. The connected length was the longest to date, but the width was very thin, approximately 1 μm (Figure 17(b)). Annealing at 573 K revealed almost no change from the pre-annealing structure, indicating that the annealing temperature was insufficient (Figure 17(c)).

[0148] Summary Cu-70 mol%Al alloy was additively manufactured using the L-PBF method, and the relative density was measured, the microstructure was observed, and the morphology change due to annealing was observed.

[0149] By measuring the relative density of the additively manufactured body and observing the microstructure, it was found that samples manufactured at high density have similar structural morphology even if the laser conditions are different.

[0150] It was suggested that by controlling the annealing time and temperature of the additively manufactured body, the CuAl2 phase becomes a continuous phase and the morphology of the α-Al phase can be controlled.

[0151] When the relative density was measured using the Archimedes method, the maximum relative density was 94.7%, and a process window was identified where the relative density exceeded 90%. Furthermore, observations revealed that the microstructural morphology of samples fabricated under laser conditions within the process window was similar.

[0152] When the additively manufactured body was annealed, the α-Al phase coarsened and the CuAl2 phase became a continuous phase as the annealing temperature and time increased. Furthermore, compared to the melt-solidified material or the metal mold cast material, the CuAl2 phase became a continuous phase throughout the entire sample, demonstrating that the Cu-Al alloy with a fine structure manufactured by the L-PBF method is suitable for morphological control of microscale structures by annealing.

[0153] (2-2) Formation of nanoscale pores by selective etching of Cu-Al alloy The annealed L-PBF specimens were selectively etched by chemical or electrochemical methods. For chemical etching, the specimens were immersed in 0.25 M hydrochloric acid for 9 h. For electrochemical etching, the specimens were immersed in 0.25 M hydrochloric acid for 9 h. The current was 7.8 mA / cm. 2 The current was passed for 5 hours at a current density of 1000 kJ / s. The structure of the specimens was observed with a scanning electron microscope (SEM) at each stage after preparation, annealing, and selective etching to investigate the changes in the structure.

[0154] Figure 18 shows a schematic diagram of the process of forming a nanoporous structure by selective etching of an L-PBF-formed body. In both chemical and electrochemical etching, the less noble metal is selectively removed. If a binary alloy is used as the initial state, the less noble elements present on the surface are removed first, leaving the more noble elements isolated. Because isolated single atoms are in a high-energy state, they attempt to change to a lower-energy state through diffusion and self-accumulation. As a result, the less noble elements present in the next layer below are exposed and selectively removed. By repeating this process, the more noble elements spontaneously form a nanoporous structure.

[0155] Experimental Method Chemical etching of additively manufactured objects The solution used for chemical etching was HCl (HCl: 0.35 wt %, manufactured by Nacalai Tesque, Inc.). Chemical etching was carried out in the following four stages.

[0156] (i) The additive manufacturing bodies No. 3 (annealed at 816 K for 24 hours) and No. 23 (annealed at 816 K for 24 hours) were immersed in 1 M HCl for 6 hours and 24 hours.

[0157] (ii) The additive manufacturing body No. 5 (annealed at 816 K for 24 hours) was immersed in 1 M HCl for 6 hours, 12 hours, and 18 hours.

[0158] (iii) The additive manufacturing bodies No. 4 (annealed at 816 K for 24 hours) and No. 10 (annealed at 816 K for 24 hours) were immersed in 1 M HCl for 3 hours and 5 hours, respectively.

[0159] (iv) No. 5 (annealed at 816 K for 12 hours) was immersed in 0.25 M HCl for 6 hours. This specimen was judged to be insufficiently etched, so it was immersed in 0.25 M HCl for an additional 3 hours, for a total of 9 hours of chemical etching.

[0160] All specimens were embedded in resin (same as in the microstructural observation of molten and solidified materials and metal mold cast materials before and after annealing) and mechanically polished before chemical etching.

[0161] Electrochemical etching of additively manufactured objects First, a preliminary experiment was conducted to measure the risk of electrochemical etching of Cu-Al alloys. In the preliminary experiment, an Al foil (1 x 5 cm, 50 μm thick) was used as the working electrode, a Cu foil (1 x 5 cm, 40 μm thick) as the counter electrode, and NaCl (1 M) as the electrolyte. A current of 1 mA was applied for 2 hours, and the presence or absence of H2 gas generation was examined. If H2 gas continues to be generated, operation inside a draft chamber is required, but in this case, H2 gas was not generated.

[0162] After confirming the safety of electrochemical etching in a preliminary experiment, electrochemical etching was carried out under the following four conditions.

[0163] (i) 1mA cm for metal mold casting materials -2 The current was passed for 5 h at a current density of 1 M NaCl as the electrolyte.

[0164] (ii) For the product No. 4 annealed at 816 K for 24 hours (solid melt solidification, metal mold casting material), the current was 7.8125 mA cm -2 The current was passed for 5 h at a current density of 1 M NaCl as the electrolyte.

[0165] (iii) For the No. 10 shaped body annealed at 816 K for 1 hour (solid melt solidification, metal mold casting material), 20 mA cm -2 A current was passed for 5 h at a current density of 1 M CuSO4.

[0166] (iv) For the product No. 5, which was annealed at 816 K for 6 hours (solid melting, metal mold casting material), 20 mA cm -2 at a current density of 10 mA cm for 1 h -2 The current was passed for 2.5 h at a current density of 1 M CuSO4.

[0167] The current was applied using a potentiostat (VMP 2 Multichannel Potentiostat, manufactured by BioLogic).

[0168] The sample used in (i) was not polished in order to examine the effect of electrochemical etching in the pre-polishing state, but all other samples were electrochemically etched after being embedded in resin (similar to the microstructural observation of molten solidified and mold cast materials before and after annealing) and mechanically polished.

[0169] Scanning electron microscope observation of additive manufacturing products after chemical etching A scanning electron microscope (SEM: JEOL Ltd. JSM-IT500) and a field emission electron microscope (FE-SEM: JEOL Ltd. JSM-7001F or ZEISS Ltd. GeminiSEM560) were used to observe the samples after chemical etching.

[0170] Scanning electron microscope observation of additive manufacturing products after electrochemical etching A scanning electron microscope (SEM: JEOL Ltd. JSM-IT500) and a field emission electron microscope (FE-SEM: JEOL Ltd. JSM-7001F) were used to observe the samples after electrochemical etching.

[0171] Results and Discussion Formation of nano- and micro-scale holes by chemical etching Figure 19 shows the results of chemical etching of additively manufactured bodies No. 3 ((a) 6 h, (b) 24 h) and No. 23 ((c) 6 h, (d) 24 h) performed using chemical etching (i). At the end of the 6-h chemical etching, microscale pores were confirmed to have formed. The shape of the microscale pores was similar to that of the α-Al phase that had aggregated during annealing, and it is believed that the microscale pores were formed as a result of the removal of the α-Al phase.

[0172] Figure 20 shows the results of chemical etching of model No. 5 ((a) 6 hours, (b) 12 hours, (c) 18 hours) performed using chemical etching of additive manufacturing models (ii). To investigate the relationship between etching time and porous structure formation in more detail, the sample surface was observed every 6 hours. Microscale pores were observed after 6 hours of etching.

[0173] Figure 21 shows the results of chemical etching of model No. 4 ((a) 3 h, (b) 5 h) and model No. 10 ((c) 6 h, (d) 10 h) performed using chemical etching of additive manufacturing models (iii). Chemical etching was performed with the immersion time shortened to 3 h and 5 h. When observed after etching, no cracks were observed on the sample surface. This suggests that there is a boundary between 5 and 6 h after chemical etching using 1 M HCl at which cracks will occur in the sample.

[0174] Chemical etching for 3 and 5 hours suppressed the formation of microscale pores compared to chemical etching for 6 hours, and the number of microscale pores was greater after 5 hours of etching than after 3 hours. Therefore, it is thought that selective removal of the α-Al phase occurs actively from the start of sample immersion until approximately 6 hours when chemical etching using 1M HCl is used. Furthermore, since the sample surfaces used in this study were all approximately 5 × 5 mm in size, it is possible that Al removal was already complete after etching for more than 6 hours.

[0175] Figure 22 shows the results of chemical etching of model No. 5 ((a) 6 h, (b) 9 h) performed using chemical etching of an additive manufacturing model (iv). SEM observations showed that chemical etching using 0.25 M HCl did not cause cracks in the sample and that Al was sufficiently removed.

[0176] Figure 22(a) shows an SEM image of the No. 5 model after etching for 6 hours. However, since there were some areas where the α-Al phase had been removed and microscale pores had formed, and other areas where this was not the case, an additional 3 hours of etching was performed.

[0177] Figure 22(b) shows an SEM image of model No. 5 after a total of 9 hours of etching. It was confirmed that the surface around the microscale holes was rougher after 9 hours of etching compared to after 6 hours of etching.

[0178] Chemical Etching of Layer-by-Layer Models (iv) A more detailed observation of model No. 5, which had been chemically etched, was carried out using a field emission electron microscope (FE-SEM: JSM-7001F manufactured by JEOL and GeminiSEM560 manufactured by ZEISS).

[0179] FIG. 23 shows the results of observing the molded body No. 5 using an FE-SEM (JSM-7001F manufactured by JEOL Ltd.).

[0180] Microscale pores with diameters of several μm (Figure 23(c)) and nanoscale pores with diameters of several tens of nm (Figure 23(d)) were confirmed. Furthermore, it was found that nanoscale pores were also formed on the inner walls of the microscale pores. This confirmed that annealing a Cu-70mol%Al alloy followed by chemical etching can produce hierarchical porous Cu with a nano- and microscale hierarchical structure.

[0181] The results of observing the shaped body No. 5 using an FE-SEM (ZEISS GeminiSEM560) are shown in Fig. 24. The nanoscale pores were observed in more detail using the FE-SEM (ZEISS GeminiSEM560).

[0182] The ZEISS GeminiSEM560 is classified as an in-lens SEM and is suitable for observing minute irregularities on the sample surface. The nanoscale pores themselves have a coral reef-like structure measuring several tens of nanometers in size, but it was found that an even finer mesh-like structure measuring several nanometers was formed on the surface (Figure 24(e)).

[0183] Formation of nano- and micro-scale holes by electrochemical etching Figure 25 shows the observation results of a sample that was electrochemically etched using electrochemical etching of an additive manufacturing body (ii) (Figure 25(a)). After etching, the α-Al phase was removed and microscale pores were formed. In this sample, gas that is thought to be H2 was generated during etching, and white crystals that are thought to be Al(OH)3 appeared on the surface of the sample after etching. The Al(OH)3 was removed with acetic acid before observation.

[0184] Al dissolved from Cu-Al alloy 3+ was supposed to be deposited on the counter electrode and collected,

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[0185] This sample was observed in more detail using a FE-SEM (JEOL JSM-7001F). As with chemical etching, microscale pores with a diameter of several μm (Fig. 25(b)) were formed, surrounded by nanoscale pores with a size of several tens of nm (Fig. 25(c)). Furthermore, nanoscale pores were formed on the inner walls of the microscale pores, confirming the formation of a hierarchical porous structure in electrochemical etching as well.

[0186] The results of electrochemical etching of the additive manufacturing body (iii) are shown below. The electrolyte was changed to CuSO4 and etching was performed. Since the standard electrode potential is higher in the order of Cu, H, and Al, when CuSO4 is used as the electrolyte,

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[0187] Figure 26 shows the SEM observation results of electrochemical etching performed in (vi) of the additive manufacturing body. CuSO4 was used as the electrolyte, and the potentiostat and Cu-Al alloy were connected with a Ni wire. The Ni wire was placed so as not to come into contact with the CuSO4 solution. Initially, a current of 20 mA cm -2 When a current was applied at a current density of 10 mA cm, the voltage became too high, so after 1 hour the current density was reduced to 10 mA cm -2 The temperature was lowered to 0°C and etching was carried out for 2.5 hours.

[0188] Summary The annealed additive manufacturing body was selectively etched using chemical or electrochemical techniques to obtain nano- and micro-scale pores.

[0189] The formation of micro- and nano-scale pores was confirmed in both chemical and electrochemical etching. Furthermore, detailed observation of the nano-scale pores formed by chemical etching revealed that a mesh-like structure on the surface of the nano-scale pores was formed on the surface.

[0190] In both methods, the diameter of the microscale pores was found to be several micrometers, while the size of the nanoscale pores was found to be several tens of nanometers. Furthermore, nanoscale pores were also formed on the inner walls of the microscale pores, confirming that the formation of a hierarchical porous structure was achieved through selective etching. Furthermore, more detailed observation of the nanoscale pores formed by chemical etching revealed that a smaller network structure, several nanometers in size, was formed on the surface of the nanoscale pores.

[0191] [2] Hierarchical porous metal [ka]

[0192] (1) L-PBF modeling (a) P is 240 W, v is 1.0 m s -1The microstructure of a Cu-70 mol% Al alloy fabricated using L-PBF is shown in Figure 1. The lamellar spacing at its finest was approximately 0.15 μm. Furthermore, when the cross sections of several high-density fabricated bodies were examined, the microstructural morphology was generally the same regardless of the laser conditions, although there were differences in the distribution and quantity of pores.

[0193] The cooling rate from the molten state of the molten solidified material is 10 -1 K·s -1 The cooling rate of the metal mold casting material is about 10 2 K·s -1 From the observation by SEM, it is clear that the α-Al phase and CuAl 2 It was confirmed that the eutectic structure consisting of the CuAl2 phase and the structure of the primary crystal CuAl2 phase after solidification differ between the melt-solidified material and the metal mold cast material. This is due to the difference in the cooling rate between the two.

[0194] In the melt-solidified material, the primary crystals of CuAl2 phase showed a rhombic shape with a side length of about 200 μm, and a eutectic structure with a width of about 2 μm was formed between the primary crystals.

[0195] In the metal mold cast material, the primary crystal CuAl2 phase exhibited a block-like shape with a side length of approximately 20 μm, and the spacing of the eutectic structure was approximately 0.5 μm.

[0196] When both materials were annealed, coarsening of the α-Al and CuAl2 phases was observed. In the fine eutectic structure of the melt-solidified material, the α-Al and CuAl2 phases aggregated, but little change was observed in the eutectic structure, which was originally about 10 μm in size.

[0197] Furthermore, significant coarsening of the CuAl2 phase and aggregation of the α-Al phase were observed in the permanent mold cast material. This is thought to be because the spacing of the eutectic structure in the permanent mold cast material is smaller than that in the melt-solidified material, making it easier for structural changes driven by interfacial energy to occur.

[0198] From the above, it was found that increasing the cooling rate of the molten metal makes the initial structure finer, and the effects of annealing are more likely to be seen.

[0199] (2) Annealing (b) Annealing the as-formed material transformed the lamellar two-phase structure of α-Al and CuAl2 phases into a structure in which the α-Al phase was dispersed within the CuAl2 phase. This indicates that the fine lamellar two-phase structure after forming has a very large interfacial area, and that annealing caused a structural change driven by interfacial energy. The three-dimensional continuity of the α-Al phase is important for the formation of micro-scale pores as material transport pathways. Depending on the alloy composition and annealing conditions, the α-Al phase can become continuous.

[0200] Using the L-PBF method, Cu-70mol%Al alloy was additively manufactured, and the relative density was measured, the microstructure was observed, and the morphology change due to annealing was observed. When the relative density was measured using the Archimedes method, the maximum relative density was 94.7%, and a process window was identified where the relative density exceeded 90%. Furthermore, observations revealed that the microstructure morphology of samples manufactured under laser conditions within the process window was similar.

[0201] When the additively manufactured body was annealed, the α-Al phase coarsened and the CuAl2 phase became a continuous phase as the annealing temperature and time increased. Furthermore, compared to the melt-solidified material and the metal mold cast material, the CuAl2 phase became a continuous phase throughout the entire sample, demonstrating that the Cu-Al alloy with a fine structure manufactured by the L-PBF method is suitable for morphological control of microscale structures by annealing.

[0202] (3) Selective etching (c) (d) Chemical etching of the annealed sample selectively removed the α-Al phase, forming microscale pores. Furthermore, nanoscale pores were formed on the Cu surface and the inner walls of the microscale pores due to the selective removal of Al from the CuAl2 phase. More detailed observation confirmed the formation of a network structure measuring several nanometers on the surface of the nanoscale pores. Electrochemical etching also produced hierarchical nano / microscale pores similar to those obtained with chemical etching.

[0203] The annealed additive manufacturing bodies were selectively etched using chemical or electrochemical methods to obtain nano- and micro-scale pores. Using both methods, the diameter of the micro-scale pores was found to be several micrometers, while the size of the nano-scale pores was found to be several tens of nanometers. Furthermore, nano-scale pores were also formed on the inner walls of the micro-scale pores, confirming the formation of a hierarchical porous structure through selective etching. Furthermore, more detailed observation of the nano-scale pores formed by chemical etching revealed that a network-like structure, several nanometers in size, was formed on the surface of the nano-scale pores.

[0204] (4) Summary The process of annealing and selective etching Cu-Al alloys fabricated by the L-PBF method is a promising method for producing hierarchically porous Cu.

[0205] By annealing and selectively etching a Cu-70mol%Al alloy with a fine structure fabricated by the L-PBF method, hierarchical porous Cu with a hierarchical structure on the nano- and micro-scale can be fabricated.

[0206] FIG. 27 illustrates the relationship between the process conditions up to the annealing step of the present invention and the microstructure.

[0207] [3] Industrial Applicability In this invention, a Cu-70mol%Al alloy is fabricated using laser powder bed fusion (L-PBF), a metal additive manufacturing method, and the shape of the μm-scale structure is controlled by annealing. After that, nm / μm-scale structures can be imparted by selective etching using chemical and electrochemical techniques.

[0208] One way to utilize nanoporous structures is to enhance boiling heat transfer. Boiling heat transfer can be enhanced by micro- or nano-scale surface modification. In the field of electrochemistry, nanoporous structures are used in lithium-ion batteries. Increasing the surface area by adding an ordered nanoporous structure to metal surfaces is required for high-surface-area conductive current collectors and catalyst materials.

Claims

1. A porous metal, It consists of a eutectic alloy, having millimeter-scale pores, micrometer-scale pores, and nanometer-scale pores; A porous metal with a hierarchical structure.

2. The porous metal according to claim 1, wherein the micrometer-scale pores have a spherical or pipe-like structure.

3. A method for producing porous metal, comprising: (1) A process for producing an additive manufacturing body of a eutectic alloy using a laser powder bed fusion (L-PBF) method; (2) annealing the additive manufacturing body obtained in step (1); and (3) etching the annealed layered manufactured body obtained in step (2); The porous metal has millimeter-scale pores, micrometer-scale pores, and nanometer-scale pores; A method for producing porous metals with a hierarchical structure.

4. The L-PBF method is a method in which the cooling rate of the molten metal is 10 5 K・s~10 7 K.s. -1 4. The method for producing a porous metal according to claim 3, wherein:

5. The method for producing a porous metal according to claim 3 , wherein the layered manufactured body has a lattice structure.

6. 4. The method for producing a porous metal according to claim 3, wherein the annealing is carried out at a temperature that is 5 K to 50 K lower than the eutectic temperature of the eutectic alloy.

7. 4. The method for producing a porous metal according to claim 3, wherein the etching is chemical etching and / or electrochemical etching.

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