Porous alumina film, porous alumina plate, and method for producing the same

Abstract

To provide a porous alumina film in which the surface structure of a porous alumina layer after anodic oxidation is maintained, and which has high corrosion resistance, to provide a porous alumina plate, and to provide a method for producing the same.SOLUTION: The porous alumina layer is provided with: a plurality of pores extending in the thickness direction from one surface of the aluminum substrate; and a protective film covering the inner surfaces of the pores and the surface between the pores adjacent to each other.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a porous alumina membrane having a plurality of pores formed therein, a porous alumina plate, and a method for manufacturing the same. [Background technology]

[0002] Many of the various aluminum products made from aluminum are subjected to anodizing and sealing treatments, primarily for the purpose of improving corrosion resistance. By applying a current to aluminum using an acidic or basic electrolyte, the aluminum is anodized to form a porous oxide film on the surface. The thickness of the oxide film formed can be controlled by the anodization time, so the oxide film can be formed to any thickness.

[0003] Although the oxide film obtained by anodization is effective in improving the corrosion resistance of aluminum materials, the resulting oxide film has a high density of pores (porous alumina layer). Furthermore, at the bottom of the pores, an aluminum oxide (alumina) layer called a barrier layer with a thickness of 1 μm or less is formed. The thickness of this barrier layer remains constant and is independent of the thickness of the porous alumina layer formed by anodization. Therefore, even if the overall thickness of the porous alumina layer is increased by anodization, each pore functions as a corrosion initiation point, making it impossible to achieve sufficient corrosion resistance.

[0004] For this reason, after anodizing the aluminum, a sealing treatment is generally performed to seal the pores formed in the porous alumina layer, for example, using boiling water, steam, or an aqueous solution containing metal salts such as Li or Na. This sealing treatment seals the pores in the porous alumina layer formed on the aluminum surface, blocking the bottoms of the thin pores and providing excellent corrosion resistance.

[0005] In this type of pore-sealing treatment, when boiling water or steam is used for hydration sealing, the surface of the porous alumina layer dissolves, and then hydrated aluminum oxide precipitates inside the pores, resulting in pore-sealing. The hydrated aluminum oxide precipitates not only inside the pores but also on the surface of the porous alumina layer, resulting in a significant change in the surface structure compared to the state immediately after anodization. Even when a pore-sealing treatment is performed using an aqueous solution containing metal salts, the surface structure of the porous alumina layer changes as the pores are sealed (see, for example, Non-Patent Document 1).

[0006] On the other hand, when such a porous alumina layer is used as a nanomaterial for a specific functional film, its surface structure can be precisely controlled by adjusting the anodization conditions and etching to enlarge the pore size, and therefore the surface properties can be controlled based on the surface structure of the porous alumina layer.

[0007] For example, the wettability of an aluminum material having a porous alumina layer formed on its surface can be controlled by the diameter of the pores (hereinafter referred to as pore size) and porosity of the porous alumina layer. Although alumina itself is a hydrophilic material, surface modification with a hydrophobic treatment agent such as alkylsilane or fluoroalkylsilane can impart hydrophobicity to the surface of the porous alumina layer. The contact angle of a water droplet on the surface of the hydrophobized porous alumina layer increases with increasing pore size. This is because the larger the pore size, the greater the porosity, making the material more resistant to water wetting. Therefore, the wettability of the surface of an aluminum material can be precisely controlled by anodizing or hydrophobizing treatment. [Prior art documents] [Non-patent literature]

[0008] [Non-Patent Document 1] "Exploring changes in the composition and nanostructure of anodic alumina films due to sealing treatment," by Sachiko Ono, Altopia, April 2024 issue (Karos Publishing) Summary of the Invention [Problem to be solved by the invention]

[0009] As mentioned above, when aluminum materials are used in practical products, it is essential to perform a sealing treatment to improve corrosion resistance. However, since the surface structure of the porous alumina layer is significantly changed by the sealing treatment, even if the surface structure is precisely controlled to form pores during anodization, as mentioned above, the precisely controlled pores are destroyed by the sealing treatment, and the surface structure cannot be maintained when the pores are sealed.

[0010] If a technology can be established to perform a sealing process while maintaining the structure of the porous alumina layer after anodization, in which the surface structure is controlled on a nanometer scale, it will be possible to realize an aluminum material with a surface coating that combines the properties of a functional film with specific functions based on the nanostructure and high corrosion resistance.

[0011] The present invention has been made in consideration of the above circumstances, and aims to provide a porous alumina film that maintains the surface structure of the porous alumina layer after anodization and has high corrosion resistance, a porous alumina plate having such a film, and a method for manufacturing the same. [Means for solving the problem]

[0012] The inventors have discovered that forming a thin film of a metal or metal oxide other than aluminum on the surface of a porous alumina layer after anodization by atomic layer deposition (ALD) suppresses the progress of the sealing treatment. If the upper part of the porous alumina layer after anodization, for example, only the upper part of the pores, is covered with the above-mentioned thin film, even when a sealing treatment is performed, the sealing treatment will proceed selectively only in the lower part of the pores that are not covered with the thin film. Therefore, even after the sealing treatment is completed, the surface of the porous alumina layer, including the upper part of the pores covered with the thin film, retains the structure after anodization. Therefore, it is possible to realize a porous alumina film or porous alumina plate that combines a precisely controlled surface structure with the corrosion resistance provided by the sealing treatment.

[0013] Based on the above findings, the present invention proposes the following means. (1) A porous alumina membrane according to a first aspect of the present invention is composed of a porous alumina layer formed by anodization, and includes a plurality of pores extending in the thickness direction from one surface, and a protective film covering the inner surfaces of the pores and the one surface between adjacent pores.

[0014] (2) In the porous alumina film according to the second aspect of the present invention, in the first aspect, the protective film material constituting the protective film is composed of a metal oxide material containing at least one of Ti, Zr, V, Nb, Ta, Cr, Mo, W, Mn, Ni, Fe, Cu, Ga, In, and Co.

[0015] (3) A porous alumina film according to a third aspect of the present invention is the porous alumina film according to the second aspect, wherein the metal oxide material is any one of TiO2, Fe2O3, WO, and Nb2O5.

[0016] (4) In the porous alumina film according to the fourth aspect of the present invention, in the first aspect, the protective film material constituting the protective film is composed of a single metal material or a metal alloy material containing at least one of Au, Pt, Ti, Cr, Ni, and Mo.

[0017] (5) A porous alumina membrane according to a fifth aspect of the present invention is any one of the first to fourth aspects, wherein the pores are arranged in a regular triangular lattice pattern or a regular quadrangular lattice pattern.

[0018] (6) A porous alumina plate according to a sixth aspect of the present invention comprises an aluminum substrate and the porous alumina film according to any one of the first to fifth aspects formed on one surface of the aluminum substrate.

[0019] (7) A seventh aspect of the present invention relates to a method for manufacturing a porous alumina plate, which is the sixth method for manufacturing a porous alumina plate, and includes at least a first anodization step of anodizing the one surface of the aluminum substrate to form the pores, a protective film formation step of depositing a protective film material on the one surface by atomic layer deposition (ALD), a second anodization step of anodizing the one surface of the aluminum substrate again to form deep pores connected to the pores, and a sealing step of sealing the deep pores formed by the second anodization step by a sealing treatment.

[0020] (8) An eighth aspect of the present invention relates to a method for manufacturing a porous alumina plate, in which, in the seventh aspect, the protective film material is TiO2, and the precursor used in the atomic layer deposition (ALD) method in the protective film formation step is tetrakis(dimethylamido)titanium. [Effects of the Invention]

[0021] According to the present invention, it is possible to provide a porous alumina film and a porous alumina plate that maintain the surface structure of the porous alumina layer after anodization and have high corrosion resistance, and a method for manufacturing the same. [Brief explanation of the drawings]

[0022] [Figure 1] 1 is an enlarged cross-sectional view of a main part of a porous alumina plate provided with a porous alumina membrane according to one embodiment of the present invention. [Figure 2] FIG. 2 is a plan view of a porous alumina plate as viewed from above. [Figure 3]1 is a flowchart showing a step-by-step method for manufacturing a porous alumina plate according to one embodiment of the present invention. [Figure 4] 1A to 1C are enlarged schematic diagrams showing a main part of a method for manufacturing a porous alumina plate according to an embodiment of the present invention in a stepwise manner. [Figure 5] 10 is a SEM photograph showing the results of an embodiment. [Figure 6] 10 is a SEM photograph showing the results of an embodiment. [Figure 7] 10 is a SEM photograph showing the results of an embodiment. [Figure 8] 10 is a graph showing the results of an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0023] Hereinafter, a porous alumina membrane, a porous alumina plate, and a manufacturing method thereof according to one embodiment of the present invention will be described with reference to the drawings. Note that the following embodiments are specifically described to better understand the gist of the invention, and unless otherwise specified, do not limit the present invention. In addition, the drawings used in the following description may show essential parts enlarged for convenience in order to make the features of the present invention easier to understand, and the dimensional ratios of each component may not necessarily be the same as in reality.

[0024] In the following embodiments, a film made of alumina having a thickness on the order of approximately μm is referred to as a film (porous alumina film), and a film made of such alumina formed on an alumina substrate having a thickness on the order of approximately more than microns (for example, on the order of mm) is referred to as a plate (porous alumina plate).

[0025] (porous alumina membrane, porous alumina plate) FIG. 1 is an enlarged cross-sectional view of a main part of a porous alumina plate provided with a porous alumina membrane according to one embodiment of the present invention. The porous alumina plate 10 has an aluminum substrate 11 and a porous alumina layer 12 that constitutes the porous alumina film 1 of this embodiment and is formed on one surface 11a of the aluminum substrate 11. The porous alumina film 1 may be, for example, an upper thin film of the porous alumina layer 12 excluding a sealing region E described below.

[0026] The aluminum substrate 11 is made of pure aluminum, for example, 4N aluminum. The aluminum substrate 11 may also be made of an aluminum alloy containing metals other than aluminum. The aluminum substrate 11 may be a thick plate having a thickness of 1 mm or more, or a thin, bendable plate having a thickness of about 0.1 mm to 0.5 mm. The aluminum substrate 11 may also be an aluminum material formed into, for example, a cylindrical shape, and its shape is not limited.

[0027] The porous alumina layer 12 is mainly composed of porous aluminum oxide (Al2O3) formed by multi-stage anodizing of the aluminum substrate 11. The porous alumina layer 12 can have various thicknesses depending on the processing conditions for multi-stage anodizing, which will be described later, but it may be formed to have an overall thickness of, for example, about 1 μm to 100 μm.

[0028] The porous alumina layer 12 has a plurality of pores 14 extending in the thickness direction t of the porous alumina plate 10, the pores 14 being formed in the direction of the surface extension of one surface 11a of the aluminum substrate 11. A protective film 15 is formed so as to cover the inner surface of each pore 14 and the surface 11a of the aluminum substrate 11 between adjacent pores 14, 14. The protective film 15 does not necessarily have to be formed on the bottom of the pores 14.

[0029] The plurality of pores 14 in this embodiment may be formed randomly at any positions, and a regular arrangement is not essential, but may preferably be arranged regularly. For example, as shown in FIG. 2, the pores 14 may be arranged in an equilateral triangular lattice pattern (ideal triangular lattice pattern).

[0030] The pores 14 are holes with a high aspect ratio, and although there are no particular limitations on the diameter (opening diameter), they are formed to be, for example, 0.01 μm or more and 2 μm or less. Furthermore, although there are no particular limitations on the depth of the pores 14 in the thickness direction t, they may be formed to be, for example, 0.5 μm or more and 1000 μm or less.

[0031] The protective film 15 is a thin film deposited by atomic layer deposition (ALD), as will be described later in the manufacturing method, and the thickness of the protective film 15 may be as large as several to several tens of atoms or molecules of the protective film material that constitutes the protective film 15. As an example, the thickness may be 1 nm or more and 100 nm or less.

[0032] The protective film material that forms the protective film 15 is made of, for example, a metal oxide, a simple metal, a metal alloy, or the like. The metal oxide may be made of a metal oxide material containing at least one of Ti, Zr, V, Nb, Ta, Cr, Mo, W, Mn, Ni, Fe, Cu, Ga, In, and Co. The metal oxide may be an oxide of one of the above metals, or may be a composite oxide of two or more metals.

[0033] Of the metal oxides that make up the protective film 15, it is preferable that the protective film 15 be made of one of TiO2, Fe2O3, WO, and Nb2O5. In this embodiment, TiO2 (titanium dioxide) is used as the protective film material that forms the protective film 15.

[0034] Among the protective film materials constituting the protective film 15, examples of elemental metals include Au, Pt, Ti, Cr, Ni, and Mo. In addition, the metal alloy may be an alloy composed of two or more metals including at least one of Au, Pt, Ti, Cr, Ni, and Mo.

[0035] The thickness of the porous alumina layer 12 is formed to be greater than the depth of the pores 14. For example, the thickness of the porous alumina layer 12 is at least twice the depth of the pores 14. The region of the porous alumina layer 12 from the bottom of the pores 14 to the Al portion of the aluminum substrate 11 that is not anodized is defined as a sealing region E. This sealing region E increases the thickness of the alumina as a barrier layer at the bottom of the pores 14.

[0036] In such a sealed region E, closed pores 16, which are traces of deep pores extending from the bottom of each pore 14 that have been sealed by a sealing treatment, may be observed. These closed pores 16 are formed by a sealing step in the manufacturing method, which will be described later.

[0037] In the porous alumina plate 10 of this embodiment configured as described above, a sealing region E made of alumina as a barrier layer of sufficient thickness is formed below the bottom of the pores 14, preventing the pores from functioning as corrosion initiation points. Therefore, the shape of the pores 14 is maintained after anodizing of aluminum, and a porous alumina plate 10 with excellent corrosion resistance can be realized.

[0038] Furthermore, each of the minute pores 14 is covered with a protective film 15 made of a protective film material such as a metal oxide or a metal, and therefore is not dissolved by acids or bases, thereby further improving corrosion resistance.

[0039] Furthermore, by controlling the pore size and arrangement of the pores 14 formed in the porous alumina layer 12, the surface of the porous alumina plate 10 can be made into a functional surface with specific functions, such as a surface that is water-repellent or oil-repellent, or a surface that is reflective to electromagnetic waves of specific wavelengths.

[0040] Furthermore, by continuously changing the diameter (opening diameter) of the pores 14 along any one of the directions of the surface extension of one surface 11a of the aluminum substrate 11, it is possible to realize a functional material that also has corrosion resistance, in which the water repellency and electromagnetic wave reflectivity change continuously in any one direction.

[0041] The porous alumina film 1 according to one embodiment of the present invention is formed by peeling off the porous alumina layer 12 described above from the aluminum substrate 11. By adhering such a functional porous alumina film 1 to the surface of any material or device, it becomes possible to impart functionality such as water repellency, oil repellency, and reflectivity to electromagnetic waves of a specific wavelength to the material or device.

[0042] Furthermore, such porous alumina membrane 1 can be supported on an appropriate substrate or the like and used as a gas separation membrane. For example, as a gas separation membrane, it can separate gases with different molecular diameters from a mixed gas containing different components according to the pore diameter of the pores 14, thereby obtaining only the desired gas.

[0043] (Method for manufacturing porous alumina plate) FIG. 3 is a flow chart showing the steps of a method for producing a porous alumina plate according to one embodiment of the present invention. In this embodiment, first, an aluminum substrate 11 is prepared. In this embodiment, a 4N aluminum plate is used as the aluminum substrate 11.

[0044] Next, as an optional step, a mask layer or an embossing plate, which is patterned after the hole formation pattern to be formed in a subsequent step, is used to process the positions where the holes are to be formed (texturing step S1). For example, when a mask layer is used, a mask material is formed on the first surface 11a of the aluminum substrate 11, leaving openings only at the positions where the holes are to be formed and covering the rest of the surface. When an embossing plate is used, a metal plate with protrusions formed in the areas corresponding to the positions where the holes are to be formed is pressed against the first surface 11a of the aluminum substrate 11, forming depressions or the like at the positions where the holes are to be formed on the first surface 11a of the aluminum substrate 11.

[0045] The texturing step S1 is optional and is not essential. By controlling the conditions for the first anodizing step S2 described below, it is possible to arrange the pores regularly without performing the texturing step S1.

[0046] Next, one surface 11a of the aluminum substrate 11 is anodized to form a porous alumina layer 12 having pores 14 (first anodization step S2). The aluminum substrate 11 can be anodized by electrolysis using a mixed solution (electrolyte) of phosphoric acid and citric acid, with the aluminum substrate 11 serving as the anode. The voltage applied during anodization can be, for example, about 30 to 50 V.

[0047] As the electrolyte for anodization, other than the mixed solution of phosphoric acid and citric acid, for example, a diluted aqueous sulfuric acid solution, an aqueous phosphoric acid solution, an aqueous oxalic acid solution, an aqueous malic acid solution, an aqueous malonic acid solution, etc. can also be used.

[0048] By this anodization, one surface 11a of the aluminum substrate 11 is anodized to form a porous alumina layer 12 made of Al2O3. If the texturing step S1 is performed at this time, the generation of pores 14 is induced, for example, from depressions formed at the positions where the pores are to be formed.

[0049] As the anodization progresses, a plurality of pores 14 extending in the thickness direction are formed in the porous alumina layer 12 (see FIG. 4(a)).

[0050] Next, a protective film material is deposited on the anodized surface 11a of the aluminum substrate 11 by atomic layer deposition (ALD) to form the protective film 15 (protective film forming step S3). In the protective film forming step S3, for example, a protective film material may be deposited at the molecular level on the anodized surface 11a of the aluminum substrate 11 using a plasma ALD apparatus.

[0051] More specifically, in this embodiment, TiO2 (titanium dioxide) is selected as the protective film material. The anodized aluminum substrate 11 is then placed on a stage in the chamber of a plasma ALD apparatus, and tetrakis(dimethylamido)titanium is introduced as a precursor and allowed to adsorb onto the anodized surface 11a of the aluminum substrate 11.

[0052] Next, after purging the chamber, an oxidizing agent, such as H2O gas (water vapor), is introduced into the chamber and reacted with the precursor to form a TiO2 film. The reaction temperature at this time may be, for example, about 150 to 200°C. By repeating this procedure, a protective film 15 made of TiO2 and having a desired thickness is formed so as to cover the pores 14 formed by anodization (see FIG. 4(b)).

[0053] In this way, by using the atomic layer deposition (ALD) method in the protective film formation process S3, it is possible to form a protective film 15 that covers the entire inner surface of the fine pores 14 with a uniform thickness, compared to methods such as CVD and PVD.

[0054] Next, one surface 11a of the aluminum substrate 11 on which the protective film 15 covering the pores 14 has been formed is anodized again (second anodization step S4). In the second anodization step S4, similar to the preceding first anodization step S2, for example, electrolysis may be performed using a mixed solution (electrolyte) of phosphoric acid and citric acid, with the aluminum substrate 11 as the anode.

[0055] By this second anodization, Al in the aluminum substrate 11 is oxidized to Al2O3 to a position deeper than the bottom of the pores 14. Furthermore, by this second anodization step S4, deep pores 24 are formed, which start from the bottom of the pores 14 formed in the first anodization step S2 and extend to a deeper position in the thickness direction t (see FIG. 4(c)).

[0056] Next, the aluminum substrate 11 that has been anodized again is subjected to a sealing treatment (sealing step S5). In this sealing step S5, the aluminum substrate 11 that has been subjected to the second anodizing step S4 is hydrated by, for example, immersing it in hot water. As a result, the volume of Al2O3 that constitutes the porous alumina layer 12 increases due to the formation of hydrates through a hydration reaction, and the deep pores 24 are filled and sealed (see FIG. 4(d)).

[0057] On the other hand, in the peripheral areas of the pores 14 formed in the first anodization step S2 that are covered with the protective film 15 in the protective film formation step S3, the hydration reaction is suppressed by the barrier function of the TiO2 film, and the shape of the pores 14 is maintained.

[0058] The porous alumina plate 10 of this embodiment can be obtained through the above steps. The porous alumina plate 10 obtained by the manufacturing method of this embodiment may have closed pores 16 formed in the sealed region E formed in the second anodizing step S4 to the sealing step S5, which are traces of sealed deep pores 24 extending from the bottom of each pore 14 by a hydration treatment.

[0059] According to the method for manufacturing a porous alumina plate of this embodiment, after forming pores 14 in the first anodization step S2, these pores 14 are covered with a protective film 15 in the protective film formation step S3, and then anodization is again performed in the second anodization step S4, followed by the sealing step S5, thereby performing the sealing process without damaging the shape of the regularly arranged pores 14, and forming a sealing region E below the bottom of the pores 14, where the thickness of the alumina is increased as a barrier layer. Therefore, the shape of the plurality of pores 14 formed in the first anodization step S2 is maintained in subsequent steps, and it is possible to manufacture a porous alumina plate 10 that is corrosion-resistant.

[0060] (Method for manufacturing porous alumina membrane) Next, an example of a manufacturing method for manufacturing the porous alumina membrane 1 using the porous alumina plate 10 manufactured by the above-described manufacturing method will be described.

[0061] First, a porous alumina plate 10 is prepared, with a porous alumina layer 12 formed on one side. The porous alumina plate 10 used here has a portion formed in the first anodizing step S2, which has pores 14 covered with a protective film 15, and a sealing region E formed in the second anodizing step S4 to the sealing step S5, each of which has a different solubility in acid, for example. As an example, this can be achieved by using different anodizing conditions in the first anodizing step S2 and the second anodizing step S4.

[0062] In the porous alumina plate 10 thus obtained, for example, the sealed region E below the bottom of the pores 14 covered with the protective film 15 has higher solubility in, for example, acid than the region above it.

[0063] Next, the porous alumina plate 10 is wet-etched by immersing the entire surface of the porous alumina plate 10 in an etchant. As the etchant, for example, an aqueous solution of a weak acid such as an aqueous solution of phosphoric acid can be preferably used.

[0064] By such wet etching, the sealing region E of the porous alumina layer 12, which has high solubility in the etchant, is selectively dissolved, and the porous alumina film 1 having the pores 14 covered with the protective film 15 is peeled off from the aluminum substrate 11 made of non-anodized Al (peeling process).

[0065] By this peeling step, only the porous alumina film 1 is peeled off from the aluminum substrate 11, and thus the porous alumina film 1 can be obtained.

[0066] In the above-described peeling step, the porous alumina plate 10 used is one that has been subjected to the sealing step S5, but it is also possible to perform the above-described peeling step using a porous alumina plate before the sealing step S5, and then perform the sealing step, thereby sealing the pores generated in the second anodizing step S4 after the peeling step. The sealing step in this case can also be performed under the same conditions as the sealing step S5 in the above-described embodiment.

[0067] Although one embodiment of the present invention has been described above, this embodiment is presented as an example and is not intended to limit the scope of the invention. This embodiment can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. This embodiment and its modifications are included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as defined in the claims. [Example]

[0068] An electrolytically polished 4N aluminum substrate was prepared. Next, a mold with protrusions formed in a regular triangular lattice pattern with a period of 1 μm was pressed against one surface of the aluminum substrate to perform a texturing process, forming depressions arranged in a regular triangular lattice pattern (ideal triangular lattice pattern) on one surface of the aluminum substrate (texturing process). Note that this texturing process can be performed as needed and is not a required process.

[0069] Next, this aluminum substrate was anodized in an electrolyte containing 0.4 M citric acid and 2 mM phosphoric acid at an applied voltage of 400 V and a bath temperature of 20°C for 5 minutes to form multiple pores arranged in a triangular lattice with a center-to-center distance of 1 μm (first anodization step). These pores were formed starting from the depressions formed in the texturing step S1.

[0070] Next, a TiO2 film (protective film) was formed on one side of the aluminum substrate that had undergone the first anodization process using a plasma ALD device with tetrakis(dimethylamido)titanium as the precursor gas and water vapor as the oxidant gas at a reaction temperature of 190°C (protective film formation process). As a result, the pores formed in the first anodization process and the surfaces between the pores were covered with a TiO2 film.

[0071] Next, the aluminum substrate that had undergone the protective film formation process was anodized again for 5 minutes in an electrolyte containing 0.4 M citric acid and 2 mM phosphoric acid at an applied voltage of 400 V and a bath temperature of 20°C (second anodization process). As a result, deeper pores were formed in the depth direction starting from the bottom of the pores formed in the first anodization process, and the thickness of Al2O3, which was the oxidized Al of the aluminum substrate, increased.

[0072] The aluminum substrate that had undergone the second anodization step was then immersed in boiling water for a sealing treatment, thereby sealing the deep pores that had been formed in the second anodization step, thereby obtaining a porous alumina plate according to the present example.

[0073] Figure 5 shows SEM photographs of the state of the porous alumina plate formed according to the example at each step. Figure 5(a) is an SEM photograph of the vicinity of one surface of the aluminum substrate after the protective film formation step, Figure 5(b) is an SEM photograph of the vicinity of one surface of the aluminum substrate after the second anodization step, and Figure 5(c) is an SEM photograph of the vicinity of one surface of the aluminum substrate after the sealing step.

[0074] The SEM photograph shown in Figure 5 confirms that the pores covered with the protective film (TiO2 film) in the protective film formation process did not lose their shape or become blocked even after undergoing the second anodization process and the sealing process, and that the shape and arrangement of the pores formed in the first anodization process were maintained.

[0075] Furthermore, the thickness of the sealed region E sealed in the sealing step was changed to 8.3 μm, 7.5 μm, and 6.5 μm, and the state of the pores was observed using an electron microscope. The results are shown as SEM photographs in Figure 6.

[0076] The results shown in FIG. 6 confirm that even if the thickness of the sealing region E is changed, that is, the oxidized portion of Al in the second anodizing step is increased, the shape of the pores is not distorted or blocked, and the shape and arrangement of the pores formed in the first anodizing step are maintained.

[0077] Furthermore, the state of the pores was observed using an electron microscope when the pore diameter (pore size) was changed to 31 nm, 47 nm, 53 nm, and 69 nm, respectively. The results are shown as SEM photographs in Figure 7.

[0078] The results shown in FIG. 7 confirm that even if the diameter (pore size) of the pores formed in the first anodization process is changed, the shape of the pores is not distorted or blocked even after the second anodization process to the sealing process, and the shape and arrangement of the pores formed in the first anodization process are maintained.

[0079] Next, we measured the relationship between the diameter of the pores (pore size) of the porous alumina plate used as the functional material and the contact angle of water. The results are shown in the graph in Figure 8.

[0080] 8, it was confirmed that the water contact angle, which is an index of water repellency, increases in proportion to the diameter of the pores (pore size). Therefore, it was confirmed that by selecting the diameter of the pores (pore size) of the porous alumina plate, it is possible to realize a porous alumina plate as a functional material with any desired water repellency. [Explanation of symbols]

[0081] 1...Porous alumina membrane 10...Porous alumina plate 11...Aluminum substrate 12...Porous alumina layer 14...pore 15...Protective film

Claims

1. A porous alumina membrane is composed of a porous alumina layer formed by anodization, and is provided with a plurality of pores extending in the thickness direction from one surface, and a protective film covering the inner surfaces of the pores and the one surface between adjacent pores.

2. The porous alumina film according to claim 1, wherein the protective film material constituting the protective film is composed of a metal oxide material containing at least one of Ti, Zr, V, Nb, Ta, Cr, Mo, W, Mn, Ni, Fe, Cu, Ga, In, and Co.

3. The metal oxide material is TiO ２ , Fe ２ O ３ , WO, Nb ２ O ５ The porous alumina membrane according to claim 2, which is any one of the above.

4. The porous alumina film according to claim 1, wherein the protective film material constituting the protective film is composed of a single metal material or a metal alloy material containing at least one of Au, Pt, Ti, Cr, Ni, and Mo.

5. The porous alumina membrane according to claim 1 , wherein the pores are arranged in a regular triangular lattice pattern or a regular tetragonal lattice pattern.

6. A porous alumina plate comprising an aluminum substrate and the porous alumina film according to claim 1 formed on one surface of the aluminum substrate.

7. The method for producing a porous alumina plate according to claim 6, a first anodizing step of anodizing the one surface of the aluminum substrate to form the pores; a protective film forming step of depositing a protective film material on the one surface by atomic layer deposition (ALD); a second anodization step of anodizing the surface of the aluminum substrate again to form deep pores communicating with the pores; and a sealing step of sealing deep pores generated by the second anodizing step by a sealing treatment.

8. The protective film material is TiO ２ The method for producing a porous alumina plate according to claim 7, wherein the precursor used in the atomic layer deposition (ALD) method in the protective film formation step is tetrakis(dimethylamido)titanium.

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