Porous sheet and method for producing porous sheet

Abstract

This porous sheet is a porous sheet in which particles of an inorganic material that is at least one among metal and ceramic, and has a first surface and a second surface facing each other. The protruding peak height Rpk1 of the first surface and the protruding peak height Rpk2 of the second surface are each at most 15 μm. The difference between the protruding peak height Rpk1 and the protruding peak height Rpk2 is at most 5 μm. The average particle diameter Dav1 of the particles of the inorganic material on the first surface and the average particle diameter Dav2 of the particles of the inorganic material on the second surface are each 5 μm to 150 μm. The difference between the average particle diameter Dav1 and the average particle diameter Dav2 is at most 10 μm.

Description

Porous sheet and method for producing porous sheet

[0001] The present disclosure relates to a porous sheet and a method for producing the porous sheet. This application claims priority based on Japanese Application No. 2025-067452 filed on April 16, 2025. All the contents described in the aforementioned Japanese application are incorporated herein by reference.

[0002] The method for producing a titanium-based porous body disclosed in Patent Document 1 includes a step of preparing a raw material powder containing fine titanium particles and titanium particles having a larger particle diameter than the fine titanium particles, a step of forming a deposited layer by depositing the raw material powder on a molding surface in a dry manner, and a step of sintering the deposited layer.

[0003] International Publication No. 2022 / 064898

[0004] The porous sheet of the present disclosure is a porous sheet in which particles of an inorganic substance that is at least one of a metal and a ceramic are bonded to each other, and has a first surface and a second surface facing each other. The protruding peak height Rpk1 of the first surface and the protruding peak height Rpk2 of the second surface are each 15 μm or less. The difference between the protruding peak height Rpk1 and the protruding peak height Rpk2 is 5 μm or less. The average particle diameter Dav1 of the inorganic particles on the first surface and the average particle diameter Dav2 of the inorganic particles on the second surface are each 5 μm or more and 150 μm or less. The difference between the average particle diameter Dav1 and the average particle diameter Dav2 is 10 μm or less.

[0005] Figure 1 is a schematic perspective view showing the porous sheet of the embodiment. Figure 2 is an enlarged view of region A in Figure 1. Figure 3 is a schematic cross-sectional view showing the porous sheet of the embodiment. Figure 4 is an explanatory diagram for explaining the method for producing the porous sheet of the embodiment.

[0006] In Patent Document 1, fine titanium particles gather below the deposited layer. As a result, titanium particles having a larger particle diameter than the fine titanium particles gather above the deposited layer. By sintering the deposited layer in this state, the lower surface of the produced sheet-shaped porous body becomes a relatively smooth surface, and the upper surface becomes a relatively rough surface.

[0007] The first and second opposing surfaces of a porous sheet are desirable to have low surface roughness, low variation in surface roughness, excellent fluid flowability, and low variation in fluid flowability.

[0008] One of the purposes of this disclosure is to provide a porous sheet in which the surface roughness of the first and second surfaces facing each other is small, the variation in surface roughness is small, and the fluid flowability is excellent and the variation in flowability is small.

[0009] The porous sheet of this disclosure has low surface roughness on the first and second surfaces facing each other, low variation in surface roughness, excellent fluid flowability, and low variation in fluid flowability.

[0010] First, the embodiments of this disclosure will be listed and described.

[0011] (1) A porous sheet according to one aspect of the present disclosure is a porous sheet in which inorganic particles, which are at least one of metal and ceramics, are bonded together, and has a first surface and a second surface facing each other. The height of the protruding peaks Rpk1 of the first surface and the height of the protruding peaks Rpk2 of the second surface are 15 μm or less. The difference between the height of the protruding peaks Rpk1 and the height of the protruding peaks Rpk2 is 5 μm or less. The average particle size Dav1 of the inorganic particles on the first surface and the average particle size Dav2 of the inorganic particles on the second surface are 5 μm or more and 150 μm or less. The difference between the average particle size Dav1 and the average particle size Dav2 is 10 μm or less.

[0012] The first and second surfaces are smooth because the heights of the protruding peaks Rpk1 and Rpk2 are 15 μm or less. The surface properties of the first and second surfaces are uniform because the difference between the heights of the protruding peaks Rpk1 and Rpk2 is 5 μm or less. The porous sheet described in (1) above has excellent fluid flowability on the first surface and on the second surface because the average particle sizes Dav1 and Dav2 satisfy the above range. The porous sheet described in (1) above makes it easy to make the voids on the first surface and on the second surface uniform because the difference between the average particle sizes Dav1 and Dav2 is 10 μm or less. In other words, the porous sheet described in (1) above makes it easy to make the fluid flowability on the first surface and on the second surface uniform. Therefore, the porous sheet described in (1) above can be suitably used, for example, as a porous transport layer (PTL) and a gas diffusion layer (GDL) in a fuel cell or a PEM (Proton Exchange Membrane) type electrolytic cell.

[0013] (2) In the porous sheet described in (1) above, the average thickness may be 0.1 mm or more and 5.0 mm or less.

[0014] The porous sheet described in (2) above can be suitably used, for example, as a porous transport layer and a gas diffusion layer. The porous sheet described in (2) above facilitates the construction of small water electrolyzers and small fuel cells.

[0015] (3) In the porous sheet described in (1) or (2) above, the void ratio may be 30% or more and 80% or less.

[0016] Porous sheets with a porosity of 30% or more allow fluids to flow easily. Porous sheets with a porosity of 80% or less are easy to handle. Furthermore, porous sheets containing metal particles and with a porosity of 80% or less are more easily made electrically conductive.

[0017] (4) In any of the porous sheets described in (1) to (3) above, the metal may be titanium, a titanium alloy, stainless steel, nickel, or a nickel alloy.

[0018] The above metals have excellent corrosion resistance. Therefore, the porous sheet described in (4) above is resistant to corrosion.

[0019] (5) Any of the porous sheets described in (1) to (4) above may include a plating layer that covers at least one of the surfaces of the inorganic particles on the first surface and the surfaces of the inorganic particles on the second surface.

[0020] The porous sheet described in (5) above is easily made to have at least one of the following properties: electrical conductivity and corrosion resistance.

[0021] (6) A method for manufacturing a porous sheet according to one aspect of the present disclosure comprises the steps of: preparing a mixed powder by mixing an inorganic powder which is at least one of a metal and a ceramic with a resin powder; dry filling the inside of a mold with the mixed powder; melting the resin powder; producing a molded body by solidifying the molten resin; removing the molded body from the mold; and sintering the molded body. The mold has a bottom surface and an opening for filling the mixed powder into a sheet.

[0022] By melting the resin powder, the inorganic powder is rearranged. Then, by solidifying the resin, the rearranged inorganic powder is held in place by the resin. Since the sheet-like molded body can be handled separately from the mold, the degree of freedom in the sintering process is increased. Furthermore, the process can proceed to the sintering process while the sheet remains in the mold, without including the removal process. Therefore, the porous sheet manufacturing method described in (6) above makes it possible to easily produce porous sheets with excellent fluid flowability and small variations in flowability.

[0023] (7) The method for manufacturing the porous sheet described in (6) above may include a step between the filling step and the melting step of applying pressure to the mixed powder inside the mold in a direction toward the bottom surface from the opening.

[0024] By applying pressure to the mixed powder inside the mold before melting the resin powder, the inorganic powder is more easily rearranged when the resin powder is melted. By applying pressure, the height of the protruding peaks, particularly on the opening side surface of the molded body, can be changed. Therefore, the method for manufacturing the porous sheet described in (7) above facilitates the production of the porous sheet of the present disclosure described above.

[0025] (8) In the method for manufacturing a porous sheet according to (6) or (7) above, the resin may be a thermoplastic resin.

[0026] Thermoplastic resins are easily melted by heating the mold and easily solidified by cooling the mold. Therefore, the method for manufacturing the porous sheet described in (8) above facilitates the production of the porous sheet of the present disclosure described above.

[0027] (9) In the method for manufacturing a porous sheet as described in (7) above, or as described in (8) above which references (7) above, the pressure may be 0.3 kPa or more.

[0028] The method for manufacturing the porous sheet described in (9) above makes it easy to produce a porous sheet in which the height Rpk1 of the protruding peaks on the first surface and the height Rpk2 of the protruding peaks on the second surface are 15 μm or less.

[0029] (10) In the method for manufacturing a porous sheet as described in (7) above, (8) above which references (7) above, or (9) above, the pressure may be 4.0 kPa or more.

[0030] The method for manufacturing the porous sheet described in (10) above can produce a porous sheet in which the height Rpk1 of the protruding peaks on the first surface and the height Rpk2 of the protruding peaks on the second surface are even smaller, specifically 5.0 μm or less.

[0031] (11) Any of the methods for manufacturing a porous sheet described in (6) to (10) above may include a step of removing the resin from the molded body by heat-treating the molded body between the step of removing and the step of sintering.

[0032] The method for manufacturing the porous sheet described in (11) above is easier to sinter than the method for sintering a molded body from which the solidified resin has been removed, because it sintersects the molded body from which the solidified resin has not been removed.

[0033] (12) In any of the methods for manufacturing a porous sheet described in (6) to (11) above, the average particle size D50 of the inorganic powder may be 5 μm or more and 150 μm or less.

[0034] The method for manufacturing a porous sheet described in (12) above makes it easy to manufacture a porous sheet in which the height Rpk1 of the protruding peaks on the first surface and the height Rpk2 of the protruding peaks on the second surface are 15 μm or less. The method for manufacturing a porous sheet described in (12) above makes it possible to manufacture a porous sheet in which the average particle size of inorganic particles on the first and second surfaces is 5 μm or more and 150 μm or less.

[0035] (13) In any of the methods for manufacturing a porous sheet described in (6) to (12) above, the average particle size D50 of the resin powder may be 5 μm or more and 150 μm or less.

[0036] Resin powders within the above range have relatively good fluidity. Therefore, resin powders tend to disperse relatively uniformly in the mixed powder.

[0037] (14) In any of the methods for producing a porous sheet described in (6) to (13) above, the content ratio of the resin powder in the mixed powder may be 3% by mass or more and 10% by mass or less.

[0038] Having a content ratio of 3% by mass or more makes it easier to distribute the molten resin throughout the inorganic powder. Therefore, when the molten resin solidifies, it is easier for the solidified resin to hold the inorganic powder. Furthermore, when the solidified resin is removed, it is easier to form uniform voids. Having a content ratio of 10% by mass or less prevents the proportion of resin powder from becoming too high. Therefore, it is easier to fill the inside of the mold with the mixed powder.

[0039] (15) Any of the methods for manufacturing a porous sheet described in (6) to (14) above may include a step of plating the surface of the sintered sheet produced by the sintering step.

[0040] The method for manufacturing the porous sheet described in (15) above makes it easy to produce a porous sheet that is excellent in at least one of electrical conductivity and corrosion resistance.

[0041] Embodiments of the porous sheet and its manufacturing method described herein will be explained with reference to the drawings. In the drawings, some parts of the structure may be exaggerated or simplified for the sake of clarity. The shapes, sizes, and positional relationships shown in each figure are represented for the purpose of clarifying the explanation and do not necessarily represent the actual shapes, sizes, and positional relationships. The same reference numerals in the figures indicate the same part or corresponding part.

[0042] [Embodiment] <Porous Sheet> The porous sheet 1 of this embodiment will be described with reference to Figures 1 to 3. In this example, the porous sheet 1 is made up of inorganic particles 2 bonded together, as shown in Figure 2. In Figure 2, hatching is applied to the inorganic particles 2 for the sake of explanation. As shown in Figure 1, the porous sheet 1 has a first surface 11 and a second surface 12 that face each other. One of the features of the porous sheet 1 of this example is that the surface roughness of the first surface 11 and the surface roughness of the second surface 12 satisfy a specific size relationship, and the average particle size of the inorganic particles 2 on the first surface 11 and the average particle size of the inorganic particles 2 on the second surface 12 satisfy a specific size relationship.

[0043] The porous sheet 1 has a three-dimensional network structure. As shown in Figure 2, the porous sheet 1 includes a plurality of inorganic particles 2 and voids 3. The inorganic particles 2 are at least one of metal particles and ceramic particles. That is, the porous sheet 1 includes the following three embodiments. The first embodiment is an embodiment in which metal particles are bonded to each other. The second embodiment is an embodiment in which ceramic particles are bonded to each other. The third embodiment is an embodiment having at least a portion where metal particles are bonded to each other and a portion where ceramic particles are bonded to each other. The third embodiment may further have a portion where the metal particles and the ceramic particles are bonded to each other. Voids 3 are formed between the inorganic particles 2. That is, the void 3 is a space surrounded by the inorganic particles 2. The voids 3 are open to the first surface 11 and the second surface 12. The voids 3 are continuous from the first surface 11 to the second surface 12. Specific examples of the metal constituting the inorganic particles 2 are titanium, titanium alloy, stainless steel, nickel, or nickel alloy. These metals have excellent corrosion resistance. Therefore, the porous sheet 1 is less likely to corrode. Specific examples of the ceramics constituting the inorganic particles 2 are boron nitride, alumina, or silica.

[0044] The protruding peak height Rpk1 of the first surface 11 and the protruding peak height Rpk2 of the second surface 12 are 15 μm or less. When both the protruding peak heights Rpk1 and Rpk2 are 15 μm or less, the first surface 11 and the second surface 12 are smooth. The difference ΔRpk between the protruding peak height Rpk1 and the protruding peak height Rpk2 is 5 μm or less. When the difference ΔRpk is 5 μm or less, the surface properties of the first surface 11 and the second surface 12 are uniform. Therefore, the porous sheet 1 in which both the protruding peak heights Rpk1 and Rpk2 are 15 μm or less and ΔRpk is 5 μm or less can be suitably used, for example, for a porous transport layer and a gas diffusion layer of a fuel cell or a PEM-type electrolytic cell. Both the protruding peak heights Rpk1 and Rpk2 may be 10 μm or less, 7.5 μm or less, 5.0 μm or less, 2.5 μm or less, or 2.0 μm or less. The difference ΔRpk may be 4.5 μm or less, 3.5 μm or less, 2.5 μm or less, 1.5 μm or less, or 1.0 μm or less.

[0045] A surface roughness measuring instrument (Mitutoyo FORMTRACER Avant S3000) is used to measure the heights Rpk1 and Rpk2 of each protruding peak. The heights Rpk1 and Rpk2 of each protruding peak conform to JIS B 0671-2:2002.

[0046] The average particle size Dav1 of inorganic particles 2 on the first surface 11 and the average particle size Dav2 of inorganic particles 2 on the second surface 12 are between 5 μm and 150 μm. The porous sheet 1 has excellent fluid flowability on both the first surface 11 and the second surface 12 because both average particle sizes Dav1 and Dav2 satisfy the above range. The difference ΔDav between the average particle size Dav1 and the average particle size Dav2 is 10 μm or less. Because the difference ΔDav is 10 μm or less, the porous sheet 1 makes it easy to make the voids 3 on the first surface 11 and the voids 3 on the second surface 12 uniform. In other words, the porous sheet 1 makes it easy to make the fluid flowability on both the first surface 11 and the second surface 12 uniform. Therefore, the porous sheet 1 can be suitably used, for example, as a porous transport layer and a gas diffusion layer. The average particle sizes Dav1 and Dav2 may be between 5 μm and 100 μm, or between 10 μm and 80 μm. The difference ΔDav may be 8 μm or less, or 5 μm or less.

[0047] Each of the average particle diameters Dav1 and Dav2 is measured as follows. Using a SEM (Scanning Electron Microscope), three or more images are acquired on each of the surfaces 11 and 12. The magnification of each image is 100 times. The size of each image is 320 μm × 240 μm. In each image, as indicated by the thick broken lines in Figure 2, the outline of each inorganic particle 2 is extracted. In Figure 2, thick broken lines are attached only to some of the inorganic particles 2. In Figure 2, for convenience of description, the thick broken lines are shown larger than the outlines of the inorganic particles 2. When extracting the outlines, if a portion where two inorganic particles 2 are bonded by sintering is observed, they are recognized as two separate particles divided at the connected portion. Further, for example, if particles that can be clearly distinguished from other inorganic particles 2 such as dust, or extremely fine particles are present, they are not regarded as one inorganic particle 2 for determining the particle diameter. The extremely fine particles herein refer to particles having a particle diameter smaller than the D10 value of the raw material powder. D10 is the particle diameter at which the cumulative volume described later reaches 10%. Using image processing software (for example, Image-Pro Plus 6.1 J manufactured by Nippon Roper Co., Ltd.), the area of each inorganic particle 2 is determined from the image with the manually extracted outlines. If the image processing software has a function for automatically extracting outlines, the outlines may be extracted automatically. The number of inorganic particles 2 for which the area is determined is 50 or more. The diameter of a circle having the same area as the obtained area is determined. On each of the surfaces 11 and 12, the average value of all the determined diameters is calculated. The average values on the respective surfaces 11 and 12 are defined as average particle diameters Dav1 and Dav2.

[0048] The average thickness of the porous sheet 1 is, for example, 0.1 mm or more and 5.0 mm or less. The porous sheet 1 having an average thickness within the above range can be suitably used for, for example, a porous transport layer and a gas diffusion layer. The porous sheet 1 having an average thickness of 0.1 mm or more is excellent in mechanical strength. Furthermore, the porous sheet 1 having an average thickness of 0.1 mm or more is easy to manufacture. The porous sheet 1 having an average thickness of 1.0 mm or less facilitates the construction of a compact water electrolysis apparatus and a compact fuel cell. The average thickness of the porous sheet 1 may be 0.1 mm or more and 2.5 mm or less, 0.1 mm or more and 1.0 mm or less, 0.2 mm or more and 0.8 mm or less, or 0.3 mm or more and 0.7 mm or less.

[0049] The average thickness is measured as follows. A commercially available micrometer is used to measure the thickness. The measurement points for the thickness are explained using the example of the case where the first surface 11 of the porous sheet 1 is rectangular. The outline of the rectangular first surface 11 is composed of a first side, a second side, a third side, and a fourth side. The first side and the second side are opposite to each other and are parallel to each other. The third side and the fourth side are the sides that connect the first side and the second side, and are opposite to each other and are parallel to each other. There are nine measurement points for the thickness. The nine measurement points are the nine intersections of three parallel first imaginary lines and three second imaginary lines perpendicular to the three first imaginary lines. The three first imaginary lines are straight lines along the first side and divide the space between the first side and the second side equally. The three second imaginary lines are straight lines along the third side and divide the space between the third side and the fourth side equally. The length between the first surface 11 and the second surface 12 at each measurement point is measured, and the average value is calculated. This average value is taken as the average thickness. If the first surface 11 is circular, it is preferable to take the largest virtual quadrilateral inscribed within the outer edge of the first surface 11, or the smallest virtual quadrilateral enclosing the outer edge of the first surface 11, and consider the four sides of the virtual quadrilateral as the first to fourth sides, thereby taking nine measurement points.

[0050] The porosity is, for example, 30% to 80%. A porous sheet 1 with a porosity of 30% or more facilitates fluid flow. A porous sheet 1 with a porosity of 80% or less is easy to handle. A porous sheet 1 containing metal particles with a porosity of 80% or less easily enhances conductivity. A porosity of 40% to 70% is also acceptable.

[0051] The porosity is measured as follows: A filler is prepared by filling the voids 3 of the porous sheet 1 with resin. A cross-section is taken by cutting this filler along the thickness of the porous sheet 1. The cross-section of the filler is polished until the cross-sections of the inorganic particles 2 are visible. The polished cross-section is observed using a scanning electron microscope (SEM). The inorganic particles 2 and the resin are separated by binarization processing of the image analysis. The area obtained by subtracting the total area of ​​the inorganic particles 2 from the area of ​​the cross-section is taken as the area of ​​the voids 3. The ratio of the area of ​​the voids 3 to the area of ​​the cross-section is calculated. This calculated ratio is taken as the porosity.

[0052] The porous sheet 1 may include a plating layer 4, as shown in Figure 3. The plating layer 4 covers at least one of the surfaces of the first surface 11 and the second surface 12. Covering at least one of the surfaces of the first surface 11 and the second surface 12 means covering at least one of the surfaces of the plurality of inorganic particles 2 on the first surface 11 and the plurality of inorganic particles 2 on the second surface 12. The plating layer 4 does not block the voids 3 on the first surface 11 and the voids 3 on the second surface 12. In this example, the plating layer 4 covers both the surfaces of the first surface 11 and the second surface 12. The plating layer 4 is formed of, for example, platinum, gold, nickel, silver, or chromium. The porous sheet 1 with the plating layer 4 is more likely to have improved electrical conductivity and corrosion resistance. The thickness of the plating layer 4 is very thin compared to the average particle sizes Dav1 and Dav2 of the inorganic particles 2. The thickness of the plating layer 4 is, for example, 0.1 μm to 0.5 μm, or 0.15 μm to 0.4 μm. Therefore, the protruding peak heights Rpk1 and Rpk2 do not substantially change depending on whether the plating layer 4 is present or not. Also, the size of the voids 3 on the surface of the first surface 11 and the voids 3 on the surface of the second surface 12 do not substantially change depending on whether the plating layer 4 is present or not.

[0053] <Method for Manufacturing a Porous Sheet> The method for manufacturing a porous sheet in this example comprises the following steps: preparation, filling, pressure application, melting, solidification, heat treatment, and sintering. In the method for manufacturing a porous sheet in this example, the pressure application and heat treatment steps are not essential. If the heat treatment step is omitted, the sintering step serves as both heat treatment and sintering. The method for manufacturing a porous sheet in this example can produce the porous sheet 1 described above. In the preparation step of this example, a mixed powder 100 containing resin powder is prepared as described later. The following will specifically describe the case where the resin is a thermoplastic resin. In addition, existing resins such as thermosetting resins or UV-curing resins can be used as the resin, to the extent that they are applicable to each step of the manufacturing method in this example. When using other resins, the melting and solidification conditions of the resin can be substituted and applied accordingly.

[0054] <<Preparation Process>> The preparation process is the process of preparing the mixed powder 100 (upper diagram in Figure 4). The upper diagram in Figure 4 shows the state after the filling process. The mixed powder 100 is a powder obtained by mixing inorganic powder and thermoplastic resin powder. The inorganic powder is at least one of metal powder and ceramic powder. That is, the mixed powder 100 is a powder obtained by mixing at least one of metal powder and ceramic powder with thermoplastic resin powder. The mixed powder 100 does not contain any liquid such as water.

[0055] Specific examples of metals that make up metal powders include titanium, titanium alloys, stainless steel, nickel, or nickel alloys. These metal powders can be used, for example, as atomized powders, ground powders, reduced powders, or electrolytic powders. Specific examples of ceramics that make up ceramic powders include boron nitride, alumina, or silica. The shape of inorganic powders is not particularly limited as long as they are granular rather than fibrous.

[0056] The average particle size D50 of the inorganic powder is, for example, 5 μm to 150 μm. The average particle size D50 is the particle size at which the cumulative volume in the volume-based particle size distribution measured by a laser diffraction particle size distribution analyzer (Microtrac MT3300EX) accounts for 50%. Having the average particle size D50 within the above range makes it easier to manufacture the porous sheet 1 of this example described above. The average particle size D50 of the inorganic powder may also be 10 μm to 100 μm, or 15 μm to 80 μm.

[0057] The D90 of the inorganic powder is, for example, 120 μm or less. D90 is the particle size at which the cumulative volume is 90%. Having a D90 of 120 μm or less for the inorganic powder makes it easier to manufacture porous sheets 1 with both protruding peak heights Rpk1 and Rpk2 of 15 μm or less. The D90 of the inorganic powder may also be 110 μm or less, 100 μm or less, or 90 μm or less. The D90 of the inorganic powder may also be 10 μm or more. That is, the D90 of the inorganic powder may be 10 μm or more and 120 μm or less, 20 μm or more and 110 μm or less, 30 μm or more and 100 μm or less, or 30 μm or more and 90 μm or less.

[0058] Thermoplastic resins are, for example, thermoplastic resins with a melting point of 80°C to 300°C. Specific examples of thermoplastic resins include polyethylene resin, polypropylene resin, polystyrene resin, polyamide resin, acrylic resin, ethylene vinyl acetate copolymer resin, or polyvinyl chloride resin. In particular, using polyethylene resin, polypropylene resin, or ethylene vinyl acetate copolymer resin as the thermoplastic resin provides the following effects: Due to its low melting point, the thermoplastic resin powder is easily melted in the melting process described later. Due to its excellent release properties, the molded body produced in the solidification process described later is easily removed from the mold 200 in the heat treatment process. Due to its excellent strength, the molded body removed from the mold 200 in the heat treatment process is easy to handle.

[0059] The average particle size D50 of the thermoplastic resin powder is, for example, 5 μm to 150 μm. Thermoplastic resin powder with an average particle size D50 within the above range has relatively good fluidity. Therefore, the thermoplastic resin powder disperses relatively uniformly in the mixed powder 100. The average particle size D50 of the thermoplastic resin powder may also be 8 μm to 120 μm, or 10 μm to 100 μm.

[0060] The content of thermoplastic resin powder in the mixed powder 100 is, for example, 3% by mass or more and 10% by mass or less. That is, the content of inorganic powder in the mixed powder 100 is, for example, 90% by mass or more and 97% by mass or less. Having a thermoplastic resin powder content of 3% by mass or more in the mixed powder 100 makes it easy to distribute the molten thermoplastic resin throughout the inorganic powder during the melting process. Therefore, when the molten thermoplastic resin is solidified during the solidification process, the solidified thermoplastic resin easily holds the inorganic powder. Furthermore, when the solidified thermoplastic resin is removed during the heat treatment process, it is easy to form uniform voids 3. Having a thermoplastic resin powder content of 10% by mass or less in the mixed powder 100 prevents the proportion of thermoplastic resin powder from becoming too high. Therefore, it is easy to fill the mixed powder 100 during the filling process. Furthermore, it is also easy to level the powder if leveling is required during the filling process. The content ratio of thermoplastic resin powder in the mixed powder 100 may be 4% by mass or more and 9% by mass or less, or 5% by mass or more and 8% by mass or less.

[0061] ≪Filling Process≫ In the filling process, as shown in the upper diagram of Figure 4, the mixed powder 100 is dry-filled into the mold 200. The upper diagram of Figure 4 shows the state after filling. Dry filling means that no liquid such as a solvent or binder is used.

[0062] The mold 200 has a lower mold 210 and an upper mold 220, as shown in Figure 4. The lower mold 210 has a recess 211. The recess 211 has a bottom surface 212 for filling the mixed powder 100 into a sheet and an opening 213 that opens to the top surface. The upper mold 220 has a protrusion 221 on its bottom surface that is inserted into the recess 211. The protrusion 221 presses the mixed powder 100 filled in the recess 211. That is, the protrusion 221 applies pressure to the mixed powder 100 in the direction from the opening 213 of the recess 211 toward the bottom surface 212. The surface roughness of the parts of the mold 200 that are in contact with the mixed powder 100, specifically the inner surface of the recess 211 and the bottom surface of the protrusion 221, is smaller than the surface roughness required for the first surface 11 and the second surface 12 of the porous sheet 1 to be manufactured.

[0063] The material of the mold 200 should be appropriately selected from the viewpoint of heat resistance, processability, thermal conductivity, etc. The material of the mold 200 is a resin or metal that has heat resistance to at least the heating temperature of the melting process. A specific example of a resin is silicone resin. A specific example of a metal is stainless steel, aluminum, or aluminum alloy.

[0064] A release agent may be applied to the parts of the mold 200 that come into contact with the mixed powder 100, specifically the inner surface of the recess 211 and the lower surface of the protrusion 221. Applying a release agent makes it easier to remove the molded body from the mold 200 during the heat treatment process described later. The release agent is a release agent that has heat resistance to the heating temperature of the melting process. For example, fluororesin-based, silicone-based, or nano-coating-based release agents can be used.

[0065] In the filling process, leveling may be performed after filling. Leveling can be done using an appropriate tool such as a spatula or a powder dispenser.

[0066] 《Pressure Application Process》 In the pressure application process, as shown in the lower diagram of Figure 4, pressure is applied to the mixed powder 100 inside the mold 200 in the direction from the opening 213 of the recess 211 toward the bottom surface 212. The black arrows in the lower diagram of Figure 4 indicate the application of pressure.

[0067] The higher the applied pressure, the smaller the heights of the two protruding peaks Rpk1 and Rpk2 of the porous sheet 1 that can be manufactured. The applied pressure is, for example, 0.3 kPa or higher. With a pressure of 0.3 kPa or higher, a porous sheet 1 with protruding peak heights Rpk1 and Rpk2 of 15 μm or less can be manufactured. The applied pressure may be 1.0 kPa or higher, 1.5 kPa or higher, 2.0 kPa or higher, 3.0 kPa or higher, 4.0 kPa or higher, or 5.0 kPa or higher. In particular, if the applied pressure is 4.0 kPa or higher, a porous sheet 1 with protruding peak heights Rpk1 and Rpk2 of 5.0 μm or less can be manufactured. There is no particular upper limit to the applied pressure. The applied pressure is, for example, 30 kPa or less. In other words, the applied pressure is between 0.3 kPa and 30 kPa, 1.0 kPa and 27 kPa, 1.5 kPa and 25 kPa, 2.0 kPa and 23 kPa, 3.0 kPa and 20 kPa, 4.0 kPa and 18 kPa, or 5.0 kPa and 15 kPa.

[0068] Pressure can be applied, for example, by placing a weight on the upper surface of the upper mold 220. Alternatively, pressure may be applied by hot pressing. In the case of hot pressing, the application of pressure to the mixed powder 100 and the heating of the molding die 200 can be performed simultaneously.

[0069] The applied pressure may be released at any point before the start of the melting process, during the melting process, after the end of the melting process and before the start of the solidification process, during the solidification process, or after the end of the solidification process and before the heat treatment process. In particular, if the applied pressure is released after the completion of the solidification process and before the heat treatment process, the porous sheet 1 in this example is easier to manufacture.

[0070] ≪Melting Process≫ In the melting process, the mold 200 is heated to melt the thermoplastic resin powder inside the mold 200. The molten thermoplastic resin spreads among the inorganic powders. The melting of the thermoplastic resin powder causes the inorganic powders to be rearranged. The melting of the thermoplastic resin may be carried out under pressure applied to the mixed powder 100 inside the mold 200. Melting the thermoplastic resin powder under pressure makes it easier for the inorganic powders to be rearranged.

[0071] The mold 200 can be heated, for example, by a constant temperature bath, a hot plate, or an infrared lamp (IR lamp). The heating temperature is above the melting point of the thermoplastic resin. The heating temperature depends on the melting point of the thermoplastic resin, but is, for example, between 100°C and 300°C, or between 140°C and 200°C. The heating time is, for example, between 5 seconds and 1000 seconds, or between 30 seconds and 600 seconds. The heating time is the time during which the heating temperature is maintained within the above range.

[0072] ≪Solidification Process≫ The solidification process involves cooling the mold 200 to solidify the molten thermoplastic resin. The solidified thermoplastic resin holds the rearranged inorganic powder. The solidification of the thermoplastic resin produces a sheet-like molded body in which the inorganic powder is held in shape by the thermoplastic resin. The solidification of the molten thermoplastic resin may be carried out while maintaining a state of applied pressure. By solidifying the thermoplastic resin while applying pressure, the rearranged inorganic powder is more easily held by the thermoplastic resin. Therefore, the method for manufacturing the porous sheet in this example can produce the porous sheet 1 described above.

[0073] The mold 200 can be cooled, for example, by placing it on a heat sink. Specifically, the lower surface of the lower mold 210 is brought into contact with the heat sink. The mold 200 is cooled until the thermoplastic resin reaches the temperature at which it solidifies. Depending on the type of thermoplastic resin, the mold 200 is cooled until the temperature of the thermoplastic resin reaches 80°C or below, or 40°C or below.

[0074] ≪Heat Treatment Process≫ The heat treatment process removes the solidified thermoplastic resin from the molded body by heat treatment. The heat treatment may be performed by removing the molded body from the mold 200 and heating the molded body, or by heating the mold 200 while the molded body remains inside the mold 200. In the former case, the mold 200 does not need to have heat resistance to the heat treatment temperature of the heat treatment process. In the latter case, the mold 200 must also have heat resistance to the heat treatment temperature of the heat treatment process. The heat treatment process removes the thermoplastic resin and produces a sheet-like molded body made of inorganic powder. Even if the thermoplastic resin is removed in the heat treatment process, the shape of the molded body can be maintained as a sheet. In particular, if the melting process and the solidification process are performed under pressure, it is easier to maintain the shape of the molded body as a sheet even if the thermoplastic resin is removed.

[0075] The heat treatment is performed, for example, with the molded body or mold 200 placed on a sintering setter. The material of the sintering setter should be appropriately selected from inorganic powders or materials that do not react with the mold 200. The sintering setter is made of, for example, carbon, carbon fiber composite material (C / C composite), boron nitride, aluminum oxide, magnesium oxide, zirconium dioxide, or yttrium oxide.

[0076] The heat treatment conditions are appropriately selected according to the type of thermoplastic resin. The heat treatment temperature is, for example, 350°C to 500°C. The heat treatment time is, for example, 2 hours or more. The heat treatment time is the time during which the heat treatment temperature is maintained within the above range. The atmosphere is, for example, an air atmosphere or an inert atmosphere. The inert atmosphere is, for example, an argon atmosphere or a nitrogen atmosphere.

[0077] ≪Sintering Process≫ The sintering process sinters the molded body. The porous sheet 1 of this example described above is manufactured by the sintering process. If the heat treatment process is performed after removing the molded body from the mold 200, sintering is performed by heating the molded body without heating the mold 200. If the heat treatment process is performed with the molded body still inside the mold 200, sintering may be performed after removing the molded body from the mold 200, or by heating the mold 200 with the molded body still inside the mold 200. If sintering is performed only on the molded body, the mold 200 does not need to have heat resistance to the sintering temperature. If sintering is performed with the molded body still inside the mold 200, the mold 200 may have heat resistance to the sintering temperature, or it may be made of a material that is removed from the porous sheet 1 by sintering.

[0078] The sintering conditions can be appropriately selected depending on the type of metal and ceramic that constitutes the inorganic powder. The sintering conditions when the inorganic powder is titanium or a titanium alloy are as follows: The sintering temperature is, for example, 700°C to 1000°C. The sintering time is, for example, 2 hours or more. The sintering time is the time during which the sintering temperature is maintained within the above range. The atmosphere is, for example, an inert atmosphere or a vacuum atmosphere. An inert atmosphere is, for example, an argon atmosphere. The atmospheric pressure of a vacuum atmosphere is, for example, 1.0 × 10⁻⁶ －３ It is less than or equal to Pa.

[0079] ≪Other Processes≫ The method for manufacturing the porous sheet in this example may further include a plating step. In the plating step, the surface of the porous sheet 1 produced by the sintering step is plated. The surface of the porous sheet 1 to be plated is at least one of the first surface 11 and the second surface 12. By the plating step, a porous sheet 1 with a plated layer 4 can be manufactured. Various plating methods can be used for the plating, such as electroplating or electroless plating.

[0080] [Test Example] In the test example, a porous sheet was manufactured using the mold 200 described with reference to Figure 4. In this example, the difference in surface roughness between the upper and lower surfaces of the porous sheet due to differences in the pressure applied to the mixed powder was evaluated. In this example, the upper surface is the surface of the upper mold 220 facing the lower surface of the convex portion 221, and the lower surface is the surface of the lower mold 210 facing the bottom surface 212 of the concave portion 211.

[0081] <Sample No. 1 to Sample No. 5> Samples No. 1 to No. 5 were prepared by performing the preparation, filling, pressure application, melting, solidification, heat treatment, and sintering processes described above. A mixed powder was prepared by mixing titanium powder and polyethylene resin powder. The titanium powder was atomized. The average particle size D50 of the titanium powder was 60 μm, and D90 was 90 μm. The average particle size D50 of the polyethylene resin powder was 25 μm. The content ratio of polyethylene resin powder in the mixed powder was 5% by mass. The mixed powder was dry-filled inside the mold. The mold was heated while pressure was applied to the mixed powder inside the mold. The applied pressure is shown in Table 1. The temperature of the mold was 140°C, and the heating time was 180 seconds. The mold was cooled while maintaining the applied pressure. The mold was cooled until the temperature of the polyethylene resin reached 40°C. The molded body was removed from the mold and heat-treated while placed on a sintering setter. The heat treatment temperature was 400°C for 2 hours in a nitrogen atmosphere. The molded body from which the thermoplastic resin had been removed was sintered. The sintering temperature was 900°C for 5 hours in a vacuum atmosphere. The average thickness of the resulting porous sheet was 0.4 μm. The average thickness was measured using the measurement method described above.

[0082] <Sample No. 100> The porous sheet of Sample No. 100 was manufactured in the same manner as Sample No. 1, except that the pressure application process was omitted. In the manufacturing process of the porous sheet of Sample No. 100, no pressure was applied to the mixed powder when melting the thermoplastic resin powder, nor was pressure applied when solidifying the thermoplastic resin. Here, "no pressure applied" means that no pressure other than atmospheric pressure was acting. In Sample No. 100, the porous sheet was manufactured without using the upper mold 220.

[0083] <Surface Roughness Measurement> The surface roughness of the top and bottom surfaces of the porous sheets of each sample was measured. A surface roughness measuring instrument (FORMTRACER Avant S3000, manufactured by Mitutoyo) was used for the measurement. Table 1 shows the measured arithmetic mean roughness Ra, maximum height roughness Rz, and protruding peak height Rpk, as well as the difference between the protruding peak height Rpk of the top surface and the protruding peak height Rpk of the bottom surface calculated from the results. The arithmetic mean roughness Ra and maximum height roughness Rz were determined in accordance with JIS B 0601:2013. The protruding peak height Rpk was determined in accordance with JIS B 0671-2:2002.

[0084]

[0085] In the porous sheets of samples No. 1 to No. 5, the height Rpk of the protruding peaks on the upper surface and the height Rpk of the protruding peaks on the lower surface were 15 μm or less. The difference between the height Rpk of the protruding peaks on the upper surface and the height Rpk of the protruding peaks on the lower surface was 5 μm or less. In particular, in samples No. 3, No. 4, and No. 5, the height Rpk of the protruding peaks on the upper surface and the height Rpk of the protruding peaks on the lower surface were 5.0 μm or less, and the difference between the height Rpk of the protruding peaks on the upper surface and the height Rpk of the protruding peaks on the lower surface was 1 μm or less.

[0086] For porous sheets No. 1 to No. 5, where a pressure of 0.3 kPa or higher was applied, the heights of the protruding peaks Rpk on the upper and lower surfaces were reduced compared to sample No. 100, where no pressure was applied, and the difference between the heights of the protruding peaks Rpk on the upper and lower surfaces was also reduced. For samples No. 3, No. 4, and No. 5, where a pressure of 4.0 kPa or higher was applied, the heights of the protruding peaks Rpk on the upper and lower surfaces were reduced even further compared to samples No. 1 and No. 2.

[0087] For porous sheets No. 1 to No. 5, the arithmetic mean roughness Ra of the top surface and the arithmetic mean roughness Ra of the bottom surface were 15 μm or less. The difference between the arithmetic mean roughness Ra of the top surface and the arithmetic mean roughness Ra of the bottom surface was 3 μm or less. The maximum height roughness Rz of the top surface and the maximum height roughness Rz of the bottom surface were 100 μm or less. The difference between the maximum height roughness Rz of the top surface and the maximum height roughness Rz of the bottom surface was 10 μm or less.

[0088] <Average Particle Size and Porosity> For each sample's porous sheet, the average particle size of titanium particles on the top surface and the average particle size of titanium particles on the bottom surface were determined using the measurement method described above. The results are shown in Table 2. The porosity of each sample's porous sheet was measured using the measurement method described above. The results are shown in Table 2.

[0089]

[0090] For samples No. 1 to No. 5, the average particle size of titanium particles on the upper surface and the average particle size of titanium particles on the lower surface were between 50 μm and 60 μm, and the difference between the average particle size of titanium particles on the upper surface and the average particle size of titanium particles on the lower surface was 5 μm or less in all cases. Therefore, the porous sheets of samples No. 1 to No. 5 have excellent fluid flowability on both the upper and lower surfaces, and the fluid flowability on both surfaces is uniform. The porosity of the porous sheets of samples No. 1 to No. 5 was between 30% and 80% in all cases.

[0091] The present invention is not limited to the configurations shown in the embodiments, but is intended to include all modifications within the meaning and scope of the claims as indicated by the claims. It should be understood that at least one configuration or feature described in each embodiment and example can be combined with other embodiments and examples, or modified in various ways.

[0092] 1. Porous sheet 11. First surface, 12. Second surface 2. Inorganic particles 3. Void 4. Plating layer 100. Mixed powder 200. Molding mold 210. Lower mold, 211. Recess, 212. Bottom surface, 213. Opening 220. Upper mold, 221. Protrusion

Claims

1. A porous sheet in which inorganic particles, at least one of metal and ceramics, are bonded together, having a first surface and a second surface facing each other, wherein the height of the protruding peaks Rpk1 of the first surface and the height of the protruding peaks Rpk2 of the second surface are 15 μm or less, the difference between the height of the protruding peaks Rpk1 and the height of the protruding peaks Rpk2 is 5 μm or less, the average particle size Dav1 of the inorganic particles on the first surface and the average particle size Dav2 of the inorganic particles on the second surface are 5 μm or more and 150 μm or less, and the difference between the average particle size Dav1 and the average particle size Dav2 is 10 μm or less.

2. The porous sheet according to claim 1, wherein the average thickness is 0.1 mm or more and 5.0 mm or less.

3. The porous sheet according to claim 1 or claim 2, wherein the porosity is 30% or more and 80% or less.

4. The porous sheet according to any one of claims 1 to 3, wherein the metal is titanium, a titanium alloy, stainless steel, nickel, or a nickel alloy.

5. The porous sheet according to any one of claims 1 to 4, comprising a plating layer covering at least one of the surfaces of the inorganic particles on the first surface and the surfaces of the inorganic particles on the second surface.

6. A method for manufacturing a porous sheet, comprising the steps of: preparing a mixed powder by mixing an inorganic powder, which is at least one of a metal and a ceramic, with a resin powder; dry filling the inside of a mold with the mixed powder; melting the resin powder; producing a molded body by solidifying the molten resin; removing the molded body from the mold; and sintering the molded body, wherein the mold has a bottom surface and an opening for filling the mixed powder in a sheet shape.

7. A method for manufacturing a porous sheet according to claim 6, comprising a step of applying pressure to the mixed powder inside the mold in a direction toward the bottom surface from the opening between the filling step and the melting step.

8. The method for manufacturing a porous sheet according to claim 6 or 7, wherein the resin is a thermoplastic resin.

9. The method for manufacturing a porous sheet according to claim 7, wherein the pressure is 0.3 kPa or more.

10. The method for manufacturing a porous sheet according to claim 7 or claim 9, wherein the pressure is 4.0 kPa or more.

11. A method for manufacturing a porous sheet according to any one of claims 6 to 10, comprising a step of removing the resin from the molded body by heat-treating the molded body between the extraction step and the sintering step.

12. The method for producing a porous sheet according to any one of claims 6 to 11, wherein the average particle size D50 of the inorganic powder is 5 μm or more and 150 μm or less.

13. The method for manufacturing a porous sheet according to any one of claims 6 to 12, wherein the average particle size D50 of the resin powder is 5 μm or more and 150 μm or less.

14. The method for producing a porous sheet according to any one of claims 6 to 13, wherein the content ratio of the resin powder in the mixed powder is 3% by mass or more and 10% by mass or less.

15. A method for manufacturing a porous sheet according to any one of claims 6 to 14, comprising a step of plating the surface of the sintered sheet produced by the sintering step.

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