JP2007169766A - Porous metallic foil and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a porous metallic foil which has the penetrating micropores made finer in submicron order and uniform in diameter and which can maintain a structure even in the absence of a substrate and a method for manufacturing the same. <P>SOLUTION: A thin film prepared by dispersing metal and hetero-phase component incompatible with the metal is formed on the substrate composed of a material incompatible with the metal by a sputtering method or vacuum vapor deposition method in such a manner that the volume fraction of the hetero-phase component attains 20 to 65% and the thickness attains ≥1 μm. The resultant thin film is subjected to heat treatment and the grain size of the metal and hetero-phase component in the thin film is adjusted and thereafter the substrate and the hetero-phase component are selectively dissolved away to obtain the porous metallic foil. Portions or the whole of the micropores included in the resulted porous metallic foil penetrate above and below in the metallic foil and the diameter of the micropores is 0.01 to 1 μm. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a porous metal foil and a method for producing the same, and more particularly to a porous metal foil having through-holes and a method for producing the same.

  A porous metal material has been conventionally used, and a plate-like or thin-film metal porous body having fine pores is widely used as a functional material. Specifically, it is widely used for various filters, porous supports, catalysts utilizing a large surface area, sheet-like electrodes and the like.

  The function of the metal porous body is strongly influenced by the surface area, pore structure, thickness, and the like. For this reason, efforts have been made in terms of manufacturing process technology and pore control technology to produce a metal porous body that is as uniform in diameter as possible and has fine through-holes and is thin.

  The metal porous body can be produced by molding or applying a fine metal powder or fiber, followed by firing. For example, in Patent Document 1, a slurry in which metal powder, resin beads, an organic binder, and a solvent are mixed is applied onto a carrier sheet, dried and then removed from the carrier sheet to form a green sheet. The green sheet is degreased and fired. In addition, a method for producing a sheet-like metal porous body by eliminating organic binder and resin beads and sintering metal powder is disclosed.

  However, since the pore structure of the porous body obtained by such a method is governed by the particle size of the starting metal powder and resin beads, it is difficult to refine the pore structure on the order of submicrons. Further, since the sheet shrinks during sintering, it is difficult to control the pore diameter. Furthermore, there is a limit to reducing the thickness of the manufacturing method.

  Thus, conventionally, it has been difficult to produce a porous metal foil having through-micropores that are refined in the order of submicron and have a uniform diameter.

  Moreover, in order to use as various filters, porous supports, etc., it is difficult to perform the required function in a state where the metal porous body and the substrate are integrated. Therefore, a plate-like or thin-film metal porous body having fine pores is required to maintain the structure without a substrate.

JP 2004-332069 A

  The present invention has been made in view of such a problem, and has fine through-holes with submicron order and uniform diameter, and further maintains the structure without a substrate. An object of the present invention is to provide a porous metal foil that can be used and a method for producing the same.

  The porous metal foil according to the present invention is a metal foil having micropores and a thickness of 1 μm or more, and a part or all of the micropores penetrates, and the diameter of the micropores is It is 0.01-1 micrometer, and the porosity by this micropore is 20-65 volume%, It is characterized by the above-mentioned. The diameter of the micropores is preferably uniform. Here, the term “uniform” means that the value obtained by dividing the maximum diameter of the micropores by the minimum diameter is 2 or less.

  The porous metal foil is preferably made of at least one selected from Ta, Nb, Ta alloy, and Nb alloy.

  In the method for producing a porous metal foil according to the present invention, a thin film in which a metal and a heterophasic component incompatible with the metal are dispersed has a volume fraction of the heterophasic component of 20 to 65% by volume and a thickness of It is formed on a substrate made of a material that is incompatible with the metal so as to be 1 μm or more, and the obtained thin film is heat-treated to adjust the particle size of the metal and the heterophasic component in the thin film. The substrate and the heterogeneous component are selectively dissolved and removed.

  In forming the thin film, it is preferable to use a sputtering method or a vacuum evaporation method as a film forming method.

  The metal is preferably at least one selected from Ta, Nb, Ta alloy, and Nb alloy. In this case, it is preferable to use Cu foil or Ag foil as the substrate.

  The heterophasic component is preferably an oxide that is thermodynamically stable with respect to the metal constituting the porous metal foil, and is preferably MgO and / or CaO, for example.

  The heterophase component is preferably a metal that is not compatible with the metal constituting the porous metal foil. For example, the metal constituting the porous metal foil may be Ta, Nb, Ta alloy, Nb. In the case of using at least one selected from the group consisting of alloys, it is preferable to use at least one selected from the group consisting of Cu, Ag, Ca, and Mg.

  The porous metal foil according to the present invention has through micropores that are refined in the submicron order and have a uniform diameter, and can maintain the structure without a substrate. Since it can be handled by itself, it can be suitably used for various filters, porous supports, catalysts using large surface areas, sheet-like electrode applications, etc., and it is suitably used for various applications as a functional porous material. be able to.

  The porous metal foil according to the present invention is a metal foil having micropores and a thickness of 1 μm or more, and a part or all of the micropores penetrates, and the diameter of the micropores is It is 0.01-1 micrometer, and the porosity by this micropore is 20-65 volume%.

  Some or all of the micropores need to penetrate the porous metal foil vertically. It is because it can use suitably for various filters, a porous support body, etc. by penetrating.

  Moreover, the diameter of a micropore needs to be 0.01-1 micrometer. In the case of micropores having a diameter of less than 0.01 μm, it is difficult to permeate the catalyst and electrolyte into the micropores, making it difficult to use for catalyst applications, sheet-like electrode applications, and the like. On the other hand, the surface area cannot be made sufficiently large if the micropores have a diameter exceeding 1 μm. The diameter of the micropores can also be obtained by measuring the pore distribution by the mercury intrusion method or gas adsorption method, but here it is confirmed by visual observation by observing a cross section perpendicular to the surface of the foil with an electron microscope or the like. It is a thing.

  Furthermore, the porosity due to the fine pores needs to be 65% by volume or less. If the porosity exceeds 65% by volume, the foil strength may be drastically reduced. On the other hand, the foil strength increases as the porosity decreases, but the basic function of the porous metal foil such as the surface area and the aperture ratio may be reduced. It is necessary to be.

  In addition, it is preferable that the diameter of a micropore is as uniform as possible. When the pore diameter is not uniform, the function of the porous foil as the support, the catalyst, and the electrode varies depending on the surface area of the porous foil surface and the depth direction of the foil. Specifically, it is preferable that the value obtained by dividing the maximum diameter of the fine holes by the minimum diameter is 2 or less.

  The thickness of the porous metal foil is required to be 1 μm or more from the viewpoint of obtaining sufficient foil strength. In addition, even if the thickness exceeds 100 μm, there is no adverse effect in terms of functionality, but production efficiency considering the film formation time and the time taken to remove foreign phase components, and applications such as porous supports, filters, electrodes, etc. In consideration of the practical thickness in the case, it is preferable to set the upper limit to 100 μm. However, depending on the application, a film thickness of 100 μm or more may be adopted.

  Next, the manufacturing method of the porous metal foil which concerns on this invention is demonstrated.

  The substrate is made of a material that is incompatible with the metal that is a constituent of the porous metal foil. As a material for forming a film on the substrate, a mixed material in which a metal that is a constituent component of the porous metal foil and a heterophasic component that is essentially incompatible with the metal is mixed and dispersed is used. Examples of a method for forming the mixed material on the substrate include a sputtering method and a vacuum evaporation method. In the case where a film is formed using a sputtering method, the mixed material is used as a sputtering target to form a film on the substrate. Then, the heterogeneous component is selectively dissolved and removed from the film formed on the substrate to obtain a porous metal foil.

  The porosity of the finally obtained porous metal foil corresponds to the volume fraction of the heterogeneous component during film formation. For this reason, in consideration of the target porosity, the film is formed after determining the ratio of the metal and the heterogeneous component which are the constituent components of the porous metal foil. Since the porosity needs to be adjusted to 65% by volume or less, the volume fraction of the heterogeneous component is also adjusted to 65% by volume or less. When the porosity exceeds 65% by volume, the foil strength is drastically decreased, and the foil may be cracked during the process or handling.

  In addition, whether or not the micropores of the finally obtained porous metal foil penetrates is influenced by the ratio of the metal constituting the porous metal foil to the heterogeneous component. That is, the foil strength increases as the porosity decreases, but if the heterogeneous component is less than 20% by volume, it is difficult to selectively dissolve and remove the heterophasic component, and it is difficult to obtain fine holes penetrating the foil. become. In addition, the basic functions of the porous metal foil such as surface area and aperture ratio may be reduced. For this reason, the volume fraction of a different phase component shall be 20 volume% or more. If the metal constituting the porous metal foil and the heterogeneous component do not coexist and a complete laminated structure is formed, the heterophasic component cannot be connected in the thickness direction of the foil, and a through hole cannot be formed.

  As described above, the diameter of the micropores needs to be 0.01 to 1 μm. For this purpose, the metal and the heterogeneous component constituting the porous metal foil are finely and uniformly distributed. It is necessary to form a film in a dispersed manner. By forming the film finely and uniformly dispersed, even if the metal and the different phase component are grown by subsequent heat treatment, it can be evenly aligned with a small particle size, and both components can be evenly dispersed. Can be made.

  A sputtering method or a vapor deposition method is suitable as a method for forming a film by finely and uniformly dispersing the metal constituting the porous metal foil and the heterophasic component. This is because the sputtering method and the vapor deposition method are excellent in obtaining a finely and uniformly mixed and dispersed film structure with good reproducibility. In addition, when using a sputtering apparatus or a vacuum evaporation apparatus having a plurality of evaporation sources to simultaneously evaporate the metal constituting the porous metal foil and the different phase components and deposit them on the substrate, to the evaporation source, There is also an advantage that the composition adjustment between the metal constituting the porous metal foil and the heterophasic component can be easily performed by adjusting the input power.

  The thickness of the film to be formed is 1 μm or more. If the film thickness is less than 1 μm, it is difficult to obtain sufficient foil strength, and the foil may be cracked during the process or handling. There is no limitation on the functional aspect of the porous metal foil with respect to the upper limit of the thickness of the film to be formed. However, the upper limit is 100 μm in consideration of the production efficiency in consideration of the film formation time, the time taken to remove the heterogeneous component, and the like, and the practical thickness in applications such as porous supports, filters, and electrodes. Theoretically, a porous metal foil of 100 μm or more can be formed, and a film thickness of 100 μm or more may be adopted depending on the application.

  Various metals can be used as the metal constituting the porous metal foil, but at least one selected from Ta, Nb, Ta alloy, and Nb alloy is preferable. Since Ta and Nb are excellent in heat resistance, they can withstand high-temperature heat treatment, and the structure control during heat treatment is easy. Moreover, it is excellent also in corrosion resistance, and nitric acid etc. can be used when selectively removing the heterogeneous component and the substrate component.

  As the substrate used for film formation, various materials that are not compatible with the metal constituting the porous metal foil and can be selectively removed can be used. In addition to the above conditions, the substrate material is determined in consideration of economics, handling properties, surface flatness and the like. For example, when the metal constituting the porous metal foil is at least one selected from Ta, Nb, Ta alloy, and Nb alloy, an Ag foil or Cu foil can be suitably used as the substrate.

  As the heterophasic component, an oxide that is thermodynamically stable with respect to the metal constituting the porous metal foil or a metal having no compatibility can be used. For example, MgO and CaO are preferable as heterogeneous components because they are thermodynamically stable to most metals and can be easily dissolved with an inorganic acid or the like. Further, when the metal constituting the porous metal foil is at least one selected from Ta, Nb, Ta alloy, and Nb alloy, Cu, Ag, Ca, and Mg do not have compatibility with Ta and Nb. It can also be used as an ingredient.

  The film obtained as described above is heat-treated for each substrate in an inert atmosphere or in vacuum to grow grains of the metal and the heterogeneous component constituting the porous metal foil. As the heat treatment is performed at a lower temperature, the grain growth is suppressed, and the pores of the finally obtained metal porous foil become finer. Conversely, the grain growth progresses as the heat treatment is performed at a higher temperature, and the pore diameter of the finally obtained porous metal foil becomes larger. The temperature and atmosphere of the heat treatment are determined in consideration of the metal constituting the porous metal foil, the heterogeneous component, the melting point and vapor pressure of the substrate, and the target pore diameter of the porous metal foil. However, at least the heat treatment temperature and atmosphere must be such that the metal, the heterogeneous component, and the substrate constituting the porous metal foil do not dissolve or volatilize. For example, when Ta or Nb is used as the constituent component of the porous metal foil and Cu is used as the heterogeneous component, the heat treatment needs to be performed at a temperature lower than the melting point of Cu (1083 ° C.). However, even if it is less than the melting point of Cu (1083 ° C.), if heat treatment is performed in a vacuum immediately below the melting point, Cu may volatilize during the heat treatment, and at the same time, crushing of pores may occur. Is preferably heat-treated in an inert atmosphere such as Ar.

  When film formation is performed by sputtering or vacuum deposition, the substrate is heated during film formation, not the heat treatment after film formation, and the metal and the heterogeneous component constituting the porous metal foil are grown. In this case, the process can be simplified.

  After adjusting the particle size by heat treatment, the substrate component and the heterogeneous component are selectively dissolved and removed, washed with pure water and dried to obtain a porous metal foil. A solution for selectively dissolving and removing the substrate component and the different phase component is selected from an acid, a chloride solution, etc. in consideration of the metal constituting the porous metal foil, the different phase component, the substrate material, and the like. For example, when the metal constituting the porous metal foil is at least one selected from Ta, Nb, Ta alloy, and Nb alloy, the substrate is Cu foil or Ag foil, and the heterogeneous component is MgO or CaO, nitric acid is used. Thus, the substrate component and the foreign phase component can be selectively dissolved and removed. The metal constituting the porous metal foil is at least one selected from Ta, Nb, Ta alloy, and Nb alloy, the substrate is Cu foil or Ag foil, and the different phase component is Cu, Ag, Ca, or Mg. In some cases, nitric acid can be used to selectively dissolve and remove the substrate component and the heterogeneous component.

  Hereinafter, the present invention will be described in more detail with reference to examples.

Example 1
Hydrogenated pulverized Nb and MgO with a purity of 99.99% were mixed at a volume ratio of 50%, hot pressed at 1400 ° C. for 1 hour at a pressure of 2452 MPa (250 kgf / cm ² ), and a target with a diameter of 60 mmφ and a thickness of 5 mm Was made. As a substrate, an electrolytic copper foil having a thickness of 10 μm, a width of 10 mm, and a length of 10 mm was used and placed in a sputtering apparatus (SPF-210H, manufactured by Anerva Corporation). Then, RF sputtering was performed in an argon atmosphere of 10 mTorr to form a film having a thickness of 2 μm. The substrate is heat-treated at 600 ° C. for 1 hour in vacuum, and then immersed in 50 vol% nitric acid for 1 hour to dissolve and remove the copper foil and MgO, washed with water and dried to obtain an Nb foil. It was.

  For the obtained Nb foil, first, the surface of the foil was observed with a stereoscopic microscope to check for cracks. Thereafter, the surface and cross section of the porous foil (cross section orthogonal to the foil) are observed with an electron microscope to confirm the size of the pore diameter, and micropores penetrating the foil (hereinafter referred to as through micropores) are formed. To see if it has been. About the size of a pore diameter, the diameter of the pore was calculated | required by visually observing the electron microscope image of the cross section orthogonal to the foil surface.

  Judgment as to whether or not through-holes are formed is based on whether or not a dense layer (a portion where a different phase component remains without being dissolved and removed) can be confirmed by observing a cross section perpendicular to the foil. The holes that were not observed were judged to be through-holes. The through micropores are formed by acid or the like penetrating from the membrane surface toward the inside to dissolve and remove the different phase components. Therefore, when the cross-section perpendicular to the foil is observed and the porous metal foil has a uniform porous structure from one side to the other side, it is different from the both sides of the membrane to the inside. It can be determined that the components are completely removed and through-holes are formed. On the contrary, when a dense layer is observed inside, it can be judged that a different phase component remains and a through-hole is not formed.

  The evaluation results of the obtained Nb foil are shown in Table 1. Observation with a stereomicroscope revealed no defects. In the electron microscope observation, the diameter of the through micropore is 0.05 to 0.1 μm, and the cross section perpendicular to the foil surface (the cross section in the direction parallel to the thickness direction of the foil) is uniformly porous. It was confirmed that

  From the above, it was confirmed that a porous Nb foil having no through holes and having through micropores having a diameter of 0.05 to 0.1 μm was obtained.

(Example 2)
Using a 6-inch target of Ta and Cu with a purity of 99.99%, using an electrolytic copper foil with a thickness of 10 μm, a width of 20 mm, and a length of 20 mm as a substrate, using a multi-source sputtering apparatus (SH-450, manufactured by ULVAC, Inc.) A Ta-60 volume% Cu film having a thickness of 9 μm was formed by simultaneous sputtering of Ta and Cu in an argon atmosphere of 10 mTorr. This was subjected to heat treatment at 800 ° C. for 1 hour together with the substrate in a vacuum, and then immersed in 50% by volume of nitric acid for 1 hour to dissolve and remove copper, washed with water and dried to obtain a Ta foil.

  Table 1 shows the results of evaluation similar to that of Example 1 for the obtained Ta foil. Moreover, the photograph of the result of having observed the cross section orthogonal to the surface of the obtained foil with an electron microscope is shown in FIG. In FIG. 1, black portions are voids, and white portions are Ta particles forming a porous body.

  From the above, it was confirmed that a porous Ta foil having no through holes and having through micropores having a diameter of 0.1 to 0.2 μm was obtained.

(Example 3)
In Example 2, the same operation was performed except that the film thickness was 2 μm and the vacuum heat treatment temperature was 400 ° C., and the obtained Ta foil was evaluated in the same manner as in Example 1. The evaluation results are shown in Table 1.

  As a result, it was confirmed that a porous Ta foil having no through holes and having through micropores having a diameter of 0.03 to 0.06 μm was obtained.

Example 4
A Ta foil was obtained by performing the same operation as in Example 2 except that the composition of the film to be formed was Ta-25 vol% Cu and the thickness of the film was 2 μm. Table 1 shows the results of evaluation similar to that of Example 1 for the obtained Ta foil.

  As a result, it was confirmed that a porous Ta foil without defects and having through micropores with a diameter of 0.1 to 0.2 μm was obtained.

(Example 5)
Using a 6 inch target of Nb and Cu with a purity of 99.99%, using an electrolytic copper foil with a thickness of 10 μm, a width of 20 mm, and a length of 20 mm as a substrate, using a multi-source sputtering apparatus (SH-450, ULVAC), 10 mTorr 20 μm of Nb-60 vol% Cu was formed by simultaneous sputtering of Nb and Cu in an argon atmosphere. This was heat treated for 1 hour at 1050 ° C. in an Ar atmosphere together with the substrate, and then immersed in 50 vol% nitric acid for 1 hour to dissolve and remove copper, washed with water and dried to obtain an Nb foil. The obtained Nb foil was evaluated in the same manner as in Example 1, and the results are shown in Table 1.

  As a result, it was confirmed that a porous Nb foil having through micropores with a crack diameter of 0.5 to 0.7 μm and free of cracks was obtained.

(Comparative Example 1)
A Ta foil was obtained in the same manner as in Example 4 except that the thickness of the film to be formed was changed to 0.5 μm. Table 1 shows the results of evaluation similar to that of Example 1 for the obtained Ta foil.

  As shown in Table 1, through-micropores having a diameter of 0.1 to 0.2 μm were formed, but some foil cracking occurred when Cu was dissolved and removed and during water washing. In addition, many micro cracks having a width of several μm were observed on the foil surface, and a porous foil capable of maintaining the structure by itself was not obtained.

(Comparative Example 2)
A Ta foil was obtained by performing the same operation as in Example 2 except that the composition of the film to be formed was Ta-70 volume% Cu. Table 1 shows the results of evaluation similar to that of Example 1 for the obtained Ta foil.

  Although through-micropores having a diameter of 0.1 to 0.2 μm were formed, some foil cracks occurred when Cu was dissolved and removed and when washed with water. In addition, many micro cracks having a width of several μm were observed on the foil surface, and a porous foil capable of maintaining the structure by itself was not obtained.

(Comparative Example 3)
A Ta foil was obtained by performing the same operation as in Example 2 except that the composition of the film to be formed was Ta-15% by volume Cu and the thickness of the film was 2 μm. Table 1 shows the results of evaluation similar to that of Example 1 for the obtained Ta foil.

  There was no defect in the obtained Ta foil, and micropores with a diameter of 0.1 to 0.2 μm were confirmed in the vicinity of the foil surface, but a dense layer was observed in the cross section of the foil and penetrated the foil. Micropores were not formed.

  In Examples 1 to 5 in which the porosity (volume ratio of heterophasic components) and the film thickness are within the scope of the present invention, there are no defects and the through micropores have a diameter in the range of 0.01 to 1 μm. A porous metal foil having the following was obtained.

  On the other hand, Comparative Example 1 in which the thickness of the film to be formed is 0.5 μm, which is smaller than 1 μm, which is the lower limit of the range of the present invention, has the same film composition as that of Example 4. Nevertheless, it was found that foil cracking occurred and there was a problem with the strength of the foil.

  Further, Comparative Example 2 in which the porosity (volume ratio of the heterogeneous component) due to the pores is 70% by volume, which is larger than 65% by volume, which is the upper limit of the range of the present invention, has a sufficient film thickness of 9 μm. Although it was thick, foil cracking occurred, and it was found that there was a problem with the strength of the foil.

  From the results of Comparative Examples 1 and 2, the porous structure is obtained when the porosity (volume ratio of heterogeneous components) due to pores exceeds the upper limit of the range of the present invention or the film thickness falls below the lower limit of the range of the present invention. However, it is considered that the foil strength is insufficient and a porous foil capable of maintaining the structure by itself cannot be obtained.

  In Comparative Example 3 in which the porosity (volume ratio of heterogeneous components) due to the pores was 15% by volume smaller than 20% by volume which is the lower limit of the range of the present invention, foil cracks were not observed and there was no problem in foil strength However, it was found that pores were formed only on the foil surface, and there was a dense phase consisting of Cu remaining inside the foil, which was problematic in terms of forming through-micropores.

It is the photograph of the result of having observed the cross section orthogonal to the surface of foil about the porous metal foil which concerns on Example 2 with the electron microscope.

Claims (11)

  A metal foil having a micropore and a thickness of 1 μm or more, wherein a part or all of the micropore penetrates, the diameter of the micropore is 0.01 to 1 μm, and The porous metal foil characterized by the porosity of the fine pores being 20 to 65% by volume.   The porous metal foil according to claim 1, wherein the micropores have a uniform diameter.   The porous metal foil according to claim 1, wherein the porous metal foil is composed of at least one selected from Ta, Nb, Ta alloy, and Nb alloy.   A thin film in which a metal and a heterophasic component incompatible with the metal are dispersed is not compatible with the metal so that the volume fraction of the heterophasic component is 20 to 65% by volume and the thickness is 1 μm or more. Forming on a substrate made of a material, heat-treating the obtained thin film to adjust the particle size of the metal and the heterogeneous component in the thin film, and then selectively dissolving and removing the substrate and the heterophasic component A method for producing a porous metal foil characterized by the above.   5. The method for producing a porous metal foil according to claim 4, wherein a sputtering method or a vacuum deposition method is used as a film forming method when forming the thin film.   The method for producing a porous metal foil according to claim 4 or 5, wherein the metal is at least one selected from Ta, Nb, Ta alloy, and Nb alloy.   The method for producing a porous metal foil according to claim 6, wherein the substrate is a Cu foil or an Ag foil.   The method for producing a porous metal foil according to any one of claims 4 to 7, wherein the heterogeneous phase component is an oxide thermodynamically stable with respect to the metal constituting the porous metal foil. .   The method for producing a porous metal foil according to claim 8, wherein the oxide is MgO and / or CaO.   The method for producing a porous metal foil according to any one of claims 4 to 7, wherein the heterophasic component is a metal having no compatibility with the metal constituting the porous metal foil.   The method for producing a porous metal foil according to claim 6 or 7, wherein the heterophasic component is at least one selected from the group consisting of Cu, Ag, Ca, and Mg.