WO2015122301A1

WO2015122301A1 - Porous metal foil and method for producing same

Info

Publication number: WO2015122301A1
Application number: PCT/JP2015/052834
Authority: WO
Inventors: 尚光井上; 近藤　和夫; 池田　裕一
Original assignee: 三井金属鉱業株式会社
Priority date: 2014-02-14
Filing date: 2015-02-02
Publication date: 2015-08-20
Also published as: TW201546333A; JP2015151580A

Abstract

Provided is a porous metal foil which has a two-dimensional network structure composed of metallic fibers that are laid unregularly, wherein each of the metallic fibers has an approximately semicircular or approximately semielliptical cross section, and wherein the aperture ratio of the porous metal foil is more than 80%. The porous metal foil according to the present invention has a high aperture ratio to such an extent that the porous metal foil can be used as a transparent conductive film, and can be produced in a large quantity by an inexpensive and simple process.

Description

Porous metal foil and method for producing the same

The present invention relates to a porous metal foil and a method for producing the same, and more specifically to a porous metal foil having a high aperture ratio suitable for use as a transparent conductive film and a method for producing the same.

Touch panels that perform input by touching the display on the display screen are widely used in smartphones and personal computers. The touch panel generally has a configuration in which a transparent conductive film is formed as an electrode on a transparent substrate such as a glass plate. The most widely used material for the transparent conductive film is ITO (indium tin oxide). However, since ITO contains indium which is a rare metal, it is expensive and accompanied by supply concerns, and the film must be formed by sputtering, which increases equipment and manufacturing costs, and further due to heat during sputtering. There is a problem that the base material may be distorted depending on the material of the base material.

In view of such circumstances, a metal transparent conductive film has been proposed as a transparent conductive film material replacing ITO. For example, Patent Document 1 (Japanese Patent Application Laid-Open No. 2012-94254) discloses a transparent conductive film in which a regular fine metal wire network and a graphene sheet are combined. Patent Document 2 (Japanese Patent No. 4610416) discloses a capacitive touch panel using a metal as a conductive film having a mesh structure. Patent Document 3 (Japanese Patent No. 5282991) discloses a transparent conductive layer containing metal nanowires. Patent Document 4 (Japanese Patent Publication No. 2012-527071) also discloses a transparent conductive layer containing nanowires, as in Patent Document 3. Patent Document 5 (Japanese Patent Laid-Open No. 2013-178550) discloses a method of manufacturing a metal fine wire sheet by vapor deposition or the like using a base material having an uneven shape. However, these technologies have various problems such as a large number of manufacturing steps and high manufacturing costs, and insufficient surface resistivity and light transmittance as a transparent conductive film, and further improvements are desired. ing.

Patent Document 6 (International Publication No. 2010/034949) discloses a conductive grid having an irregular pattern manufactured using a mask having a network of openings having substantially vertical mask zone edges. It is disclosed. The conductive grid is said to have low electrical resistance (<2Ω) and high light transmittance (> 80%), but the masking layer is dried to form a mask with a network of openings. In addition, a complicated process such as a process for removing the masking layer must be performed, and the removed masking layer cannot be reused, resulting in a large number of manufacturing processes and a high manufacturing cost. Further, since the masking layer is formed each time, it is understood that the reproducibility of the network shape obtained through the masking layer is inferior. Moreover, since it is produced through masking, the cross-sectional shape of the strands constituting the conductive grid becomes substantially rectangular due to the edge of the substantially vertical mask zone.

On the other hand, porous metal foils have been proposed assuming a negative current collector of a lithium ion battery as a main application. For example, Patent Document 7 (Japanese Patent No. 4762368) discloses a porous metal foil having a two-dimensional network structure in which metal fibers are irregularly stretched. However, the aperture ratio of the porous metal foil disclosed in this document cannot be said to be sufficiently high, and is not suitable for the use of a transparent conductive film in which a light transmittance of 85% or more is desired.

JP 2012-94254 A Japanese Patent No. 4610416 Japanese Patent No. 5282991 Special table 2012-527071 gazette JP 2013-178550 A International Publication No. 2010/034949 Japanese Patent No. 4762368

The present inventors have now found that it is possible to provide a porous metal foil that has an opening ratio that is high enough to be used as a transparent conductive film and that can be mass-produced by an inexpensive and simple process.

Therefore, an object of the present invention is to provide a porous metal foil that has an opening ratio that is high enough to be used as a transparent conductive film and that can be mass-produced by an inexpensive and simple process.

According to one aspect of the present invention, the porous metal foil has a two-dimensional network structure composed of metal fibers that are irregularly stretched, and the metal fibers have a substantially semi-circular or substantially semi-elliptical cross-sectional shape. A porous metal foil is provided wherein the porous metal foil has an open area ratio greater than 80%.

According to another aspect of the present invention, a method for producing a porous metal foil comprising:
Forming a release layer made of chromium, a chromium alloy and / or a chromium oxide by performing electrolytic chromium plating on the conductive substrate, and generating a crack in the release layer by the stress of the release layer itself;
The release layer is electroplated with a metal that can be preferentially deposited on the cracks, and an infinite number of metal particles are grown along the cracks, thereby comprising a two-dimensional network structure composed of metal fibers and 80 Forming a porous metal foil having an open area ratio greater than%,
A manufacturing method is provided comprising:

It is an upper surface schematic diagram of an example of the porous metal foil by this invention. It is a schematic cross section of the metal fiber which comprises the porous metal foil by this invention. It is a figure which shows the flow of the manufacturing process of the porous metal foil by this invention. 3 is an FE-SEM image obtained by observing the porous metal foil of Sample 1 manufactured in Example 1 from directly above. 3 is an FE-SEM image obtained by magnifying and observing the metal fibers constituting the porous metal foil of Sample 1 produced in Example 1. FIG. 3 is an FE-SEM image obtained by observing the porous metal foil of Sample 3 prepared in Example 1 from directly above. 3 is an FE-SEM image obtained by magnifying and observing the metal fibers constituting the porous metal foil of Sample 3 prepared in Example 1. FIG. 4 is a SIM image measured at an inclination angle of 60 ° C. showing a cut surface of a metal fiber constituting the porous metal foil of Sample 2 produced in Example 1 cut vertically. 4 is a SIM image measured at an inclination angle of 60 ° C. showing a cut surface of a metal fiber constituting the porous metal foil of Sample 3 produced in Example 1 that is cut perpendicularly. 4 is a SIM image measured at an inclination angle of 60 ° C. showing a cut surface of a metal fiber constituting the porous metal foil of Sample 4 produced in Example 1 cut vertically. 4 is a photograph of the porous metal foil of Sample 3 peeled off with an adhesive tape in Example 3. 4 is a photograph of a porous metal foil of Sample 3 that was immersed in a Cr etching solution and peeled in Example 3. 6 is a graph showing the relationship between the aperture ratio measured in Example 4 and electrolytic copper plating time. 6 is a graph showing the relationship between the wire diameter of a metal fiber measured in Example 4 and the electrolytic copper plating time. 6 is a diagram showing light transmittance profiles in a visible light region of metal foils having various aperture ratios measured in Example 4. FIG.

Porous Metal Foil FIG. 1 shows a schematic top view of an example of a porous metal foil according to the present invention. As shown in FIG. 1, a porous metal foil 10 according to the present invention has a two-dimensional network structure composed of metal fibers 11 that are irregularly stretched. Since this two-dimensional network structure presents a unique pattern reminiscent of the mask melon skin pattern, the applicant refers to this kind of porous metal foil as a mask melon foil. And the porous metal foil 10 of this invention has an aperture ratio exceeding 80%. This extremely high aperture ratio exceeding 80% enables the high light transmittance desired for the transparent conductive film (particularly, the transmittance in the visible light region). Metallic transparent conductive films are already known (see, for example, Patent Documents 1 to 6). However, as far as the present inventors know, a transparent transparent irregular two-dimensional network structure produced by a method derived from electrolytic copper foil is known. The conductive film has not existed until now. Actually, although Patent Document 7 discloses a porous metal foil having an irregular two-dimensional network structure, the aperture ratio is low, for example, 28% or 33%, and is not suitable for use as a transparent conductive film. Met. However, on the other hand, since the porous metal foil of Patent Document 7 can be manufactured by the same technique as the electrolytic copper foil except that it involves the formation of a release layer in which cracks are formed, a transparent conductive film is manufactured by this technique. If it can be done, it can be said that it is extremely convenient from the viewpoint of manufacturing cost, mass productivity and the like. Because, unlike other conventional manufacturing methods that tend to increase the manufacturing cost due to the large number of manufacturing steps, the transparent conductive film can be mass-produced by an inexpensive and simple process according to the manufacturing method derived from electrolytic copper foil. Because it becomes. Actually, it has never been easy to obtain a porous metal foil having an open area ratio exceeding 80% by the above approach derived from electrolytic copper foil. However, the present inventors now have an open area ratio exceeding 80%. The present invention succeeded in producing a porous metal foil having an irregular two-dimensional network structure. That is, according to the present invention, it is possible to provide a porous metal foil that has an opening ratio that is high enough to be used as a transparent conductive film and that can be mass-produced by an inexpensive and simple process. Moreover, since the porous metal foil 10 is made of metal, it can have a low sheet resistance suitable for a transparent conductive film.

This porous metal foil 10 has an opening ratio of more than 80%, preferably 83% or more, more preferably 85% or more, further preferably 87% or more, particularly preferably 90% or more, 93% or more or 95%. That's it. The higher the aperture ratio, the higher the light transmittance. In particular, in porous metal foils, the aperture ratio and light transmittance (especially the light transmittance in the visible light region) have a high correlation regardless of the aperture ratio, and the aperture ratio value and the visible light region Is approximately the same as or not very close to the value of light transmittance at a wavelength of. Therefore, by having a high aperture ratio as described above, the porous metal foil 10 can have a high light transmittance (particularly the light transmittance in the visible light region), and the conductivity of the metal constituting the metal fiber 11 can be improved. Combined with, it becomes a very useful foil for use as a transparent conductive film. Since the aperture ratio is desired to be high in this way, the upper limit is not particularly limited as long as desired conductivity is ensured, but the aperture ratio is realistically 98% or less, 97% or less, or 96% or less. . The aperture ratio in the present invention is defined as an area aperture ratio, and is specifically measured by the following procedure. That is, an enlarged photograph of a certain area is taken from directly above with an electron microscope, and this is measured by calculating the ratio of the opening area to the area using image analysis software.

The metal fiber 11 is a metal fiber, and the metal to be used may be appropriately determined according to the intended use, and is not particularly limited. Preferred metals comprise at least one selected from the group consisting of copper, gold, silver, nickel, cobalt, tin, and zinc. Here, “comprising” may be any metal or alloy mainly containing the above-listed metal elements, and means that it is allowed to contain other metal elements and unavoidable impurities as the balance. Preferably, it means that 50% by weight or more of the metal or alloy is composed of the metal elements listed above, and typical examples include those composed of the metal elements listed above and inevitable impurities. These definitions shall apply equally to the same kind of expressions described below for metals. Among these metals, those suitable for the transparent conductive film include at least one selected from the group consisting of copper, copper alloys, gold, silver, nickel, cobalt, tin, and zinc, and more preferably Copper from the viewpoint of conductivity. The metal fiber may be a surface-treated metal fiber as a base material with a surface treatment agent containing a different type of metal from the base material. Examples of metals used for such surface treatment include nickel, cobalt, Tin and zinc are mentioned.

The wire diameter of the metal fiber 11 is preferably 14 μm or less, more preferably 10 μm or less, still more preferably 7 μm or less, and most preferably 4 μm or less. The thin wire diameter contributes to a high aperture ratio. For this reason, the lower limit of the wire diameter is not particularly limited as long as desired conductivity is ensured, but from the viewpoint of handling properties, the wire diameter is preferably 1 μm or more, more preferably 2 μm or more. The “wire diameter” is defined as the width (thickness) of the metal fiber 11 when the porous metal foil 10 is viewed from directly above, and includes a field emission scanning electron microscope (FE-SEM), a scanning ion microscope ( (SIM) or the like.

The metal fiber 11 has a substantially semicircular or substantially semi-elliptical cross-sectional shape as shown in FIG. This substantially semicircular or substantially semi-elliptical cross-sectional shape is a shape imparted from the manufacturing method of the present invention as shown in FIG. 3 to be described later, but undergoes masking as disclosed in Patent Document 6. Compared to a metal fiber having a rectangular cross-sectional shape to be manufactured, there is an advantage that the surface is smoother because various corners are taken and various treatments are easily performed with high accuracy. For example, when an antireflection layer is provided on the transparent conductive film, the antireflection coating agent can be sufficiently spread over the entire side surface portion of the metal fiber. Moreover, the blackening process (for example, the process of attaching copper oxide) of the metal foil can be performed uniformly without any unevenness. Furthermore, there is also an advantage that there is little reflection of transmitted light on the side surface of the metal fiber, and a decrease in display brightness can be reduced.

As shown in FIG. 1, the metal fiber 11 is preferably a branched fiber, and the porous metal foil 10 is made high by forming a two-dimensional network structure in which the branched fibers are irregularly stretched. It is possible to preferably maintain a peelable foil form while having an aperture ratio. In particular, the two-dimensional network structure preferably has an irregular shape due to cracks formed on the surface of the substrate.

It can be said that the above-described cross-sectional shape and branched shape of the metal fiber 11 are formed by connecting innumerable metal particles due to nucleation along cracks of the release layer described later. . However, since it is desirable that the adjacent metal particles are closely bonded to each other by particle growth in order to constitute the metal fiber, the metal particles constituting the metal fiber may no longer have a complete particle shape. In addition, the metal particles constituting the metal fiber 11 are continuous in a shape having a bead-like or worm-like (caterpillar-like) seam, but may be in a shape in which the seam is not substantially observed. Therefore, as shown in FIG. 2, the metal particles constituting the metal fiber 11 have a hemispherical shape having a spherical portion 11a and a bottom portion 11b, and the bottom portions 11b of all the metal particles are on the same base surface. It can also be expressed that the spherical portions 11a of all the metal particles are located on the same side with respect to the base surface. In this case, the width D of the bottom portion 11b along the basal plane becomes the wire diameter, and the maximum cross-sectional height H of the spherical portion 11a corresponds to the thickness of the porous metal foil. The basal plane and the bottom portion 11b positioned thereon reflect the planar shape of the release layer used during manufacturing. As a result, as described above, the metal fiber 11 has a substantially semicircular or substantially semi-elliptical cross-sectional shape. In the metal fiber 11, the average ratio of the maximum cross-sectional height H to the wire diameter D is not particularly limited.

According to a preferred embodiment of the present invention, the metal fiber 11 has a substantially semicircular cross-sectional shape, and the average ratio (H / D) of the maximum cross-sectional height H to the wire diameter D is typically 0.30 to It can be 0.70, more typically 0.40 to 0.60, even more typically 0.45 to 0.55, and most typically about 0.50. This average ratio can be adjusted by appropriately changing the plating conditions and the like.

According to another preferred embodiment of the present invention, the metal fiber 11 has a substantially elliptical cross-sectional shape, and the average ratio (H / D) of the maximum cross-sectional height H to the wire diameter D exceeds 0.50. It is preferably 0.50 to 2.00, more preferably 0.75 to 1.75, and particularly preferably 1.00 to 1.50. With such a ratio, the metal fiber 11 has a raised shape higher than the semicircular cross section, and the ease of peeling of the porous metal foil from the release layer after electrodeposition is improved, and the sheet resistance of the porous metal foil is improved. Is reduced. Such a shape can be realized by adding an additive to the plating bath of the porous metal foil and / or lengthening the electrolysis time.

The porous metal foil 10 preferably has a thickness of 0.5 to 28 μm, more preferably 0.75 to 17.5 μm, still more preferably 1.5 to 12.5 μm, and particularly preferably 1.75 to 10 μm. Most preferably, the thickness is 2 to 6 μm. Within this range, handling is relatively easy while the aperture ratio is high, and sheet resistance can be reduced. However, the handling of the porous metal foil 10 may be a self-supporting form peeled from the base material, may be a form as it is coated on the base material, or may be from the base material to another base material. The thickness of the porous metal foil 10 may be appropriately set within the above range according to the form to be adopted. Since the porous metal foil of the present invention has a two-dimensional network structure composed of metal fibers, the thickness of the porous metal foil corresponds to the maximum cross-sectional height of the metal fibers. Such a thickness is preferably measured by a commercially available film thickness measuring device using a measuring element larger than the pore size of the porous metal foil.

Production Method A preferred production method of the porous metal foil according to the present invention will be described. This manufacturing method includes (1) a step of preparing a conductive base material, (2) a step of forming a cracked release layer by electrolytic chrome plating, and (3) a step of forming a porous metal foil by electrolytic plating. And (4) a step of peeling the porous metal foil as desired. Since this manufacturing method is a method derived from electrolytic copper foil, the transparent conductive film can be mass-produced by an inexpensive and simple process. Therefore, although the manufacturing method of porous metal foil is preferably performed by a continuous manufacturing method, it may be performed by a single wafer method. In particular, once a cracked release layer is formed on a conductive substrate, the release layer / conductive substrate can be reused thereafter. It becomes a possible manufacturing method, and a significant reduction in manufacturing cost can be realized.

(1) Preparation of conductive substrate FIG. 3 shows a flow of a manufacturing process of a porous metal foil according to the present invention. In the production method of the present invention, first, a conductive substrate 12 is prepared as a support for producing a porous metal foil. The conductive substrate may be a substrate having conductivity that can be plated, and any of inorganic materials, organic materials, laminates, and materials having a metal surface can be used. Is a metal. Preferred examples of such metals include metals such as copper, nickel, cobalt, iron, chromium, tin, zinc, indium, silver, gold, aluminum, and titanium, and alloys containing at least one of these metal elements. More preferable are copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, and stainless steel. The form of the conductive substrate is not limited, and various forms of substrates such as a foil, a plate, and a drum can be used. In the case of a drum, a conductive metal plate may be wound around the drum body, and the thickness of the conductive metal plate in this case is preferably 1 to 20 mm. The conductive base material supports the manufactured porous metal foil during its processing or until just before its use, thereby improving the handleability of the porous metal foil. In particular, using a metal foil or a metal plate as a conductive substrate means that the metal foil or metal plate as the conductive substrate can be reused as it is after the production of the porous metal foil, or can be recycled by melting and forming the foil. It is preferable because of its advantages. In that case, it is preferable that the thickness of the metal foil or metal plate is 10 μm to 1 mm, because it is possible to secure a strength that does not cause twisting in the manufacturing process of the metal foil or metal plate and the subsequent processing / conveying process. .

In addition, prior to the formation of the release layer 13 described later, it is preferable that the conductive substrate 12 is subjected to pretreatment such as acid washing and degreasing to clean its surface.

(2) Formation of cracked release layer by electrolytic chromium plating Electrolytic chromium plating is applied to the conductive substrate 12 to form a release layer 13 made of chromium, a chromium alloy and / or a chromium oxide. A crack 13a is generated in the release layer 13 by its own stress. The peeling layer 13 is a layer for facilitating the peeling of the porous metal foil 10 to be formed on the peeling layer 13, can generate the crack 13 a, is easily plated with the crack 13 a, and has no crack. A material having the property of being difficult to be plated at 13b is used, and specifically, chromium, a chromium alloy and / or a chromium oxide. That is, a material capable of preferentially depositing a certain kind of metal in the generated crack 13 a by plating is used as the release layer 13. In particular, by using electrolytic chrome plating, the formation conditions of the release layer 13 can be easily controlled, and as a result, how the cracks 13a enter the release layer 13 can be skillfully controlled. In this way, the generation ratio of the metal fibers formed along the cracks can be controlled to be low, whereby a porous metal foil having an opening ratio exceeding 80% can be realized. The release layer may be formed in multiple layers. In this case, cracks may be formed only in the upper layer, and cracks may be formed not only in the upper layer but also in layers below it. It may be a thing. Further, diamond-like carbon (DLC) or the like may be present on the surface of the release layer. The crack 13a is preferably controlled so as to be naturally generated by the stress of the release layer 13, and need not be formed at the same time as film formation, and is generated in the subsequent cleaning and drying process, machining, etc. Good. Cracks are usually undesirable, but the manufacturing method of the present invention is rather characterized by actively utilizing them. In particular, since cracks are usually formed such that branched lines are stretched around in a two-dimensional network, porous metal foil having an extremely high aperture ratio can be formed by forming metal fibers along the cracks. Can be obtained. In addition, since the occurrence of cracks is always a concern in normal film formation processes, the generation of cracks themselves is well-experienced by those skilled in the art engaged in film formation. It is possible to easily select within the range. For example, cracks may be generated by controlling the composition of the plating bath, the thickness of the release layer, the current density, the bath temperature, the stirring conditions, the post heat treatment, or the like.

The peeling layer 13 is a chromium plating layer made of chromium, a chromium alloy and / or a chromium oxide. Chromium has a high hardness and is excellent in terms of continuous peelability, durability and corrosion resistance, and is advantageous in that it is easily peeled off due to the formation of a passive state. The thickness of the release layer 13 is preferably 4 to 120 μm, more preferably 6 to 80 μm, still more preferably 8 to 60 μm, and most preferably 10 to 40 μm. With such a composition and thickness, the porous metal foil 10 to be formed on the layer can be formed on the layer by making the release layer have a high resistance with respect to the conductive substrate while allowing the generation of cracks. It becomes easy to form and peel. However, the thickness range of the release layer 13 described above can be unnecessarily thick when considering only the releasability, but by using a thick release layer, the generation ratio of cracks is reduced. The aperture ratio of the obtained porous metal foil 10 can be significantly increased. Although the reason for this is not necessarily clear, it is considered that internal stress or internal strain is likely to be accumulated by increasing the thickness of the release layer 13, and as a result, easy generation of cracks is suppressed.

Preferable chromium plating solutions for electrolytic chromium plating include a sergeant bath (composition: chromic anhydride 250 g / L and sulfuric acid 2.5 g / L) and a hard chromium plating bath. Examples of commercially available hard chrome plating baths include Anchor 1127 manufactured by Meltex, HEEF-25 manufactured by Atotech, and Mac 1 manufactured by Nihon McDermid. Of these, the Sargent bath is particularly preferred because it tends to generate relatively few cracks and easily increases the aperture ratio of the porous metal foil. In fact, according to the knowledge of the present inventors, the Sargent bath has fewer cracks than the HEEF bath containing the additive (for example, a bath containing chromic acid, sulfuric acid and HEEF-25), and thus has a high opening ratio. can get. Electrolytic chrome plating may be carried out so as to obtain a desired thickness by appropriately setting the electrolysis conditions according to the composition of the chrome plating bath to be used. However, it is performed for 20 minutes or more at a current density of 30 to 100 A / dm ^2. More preferably, it is 25 minutes or more at a current density of 40 to 90 A / dm ² , more preferably 30 minutes or more at a current density of 45 to 70 A / dm ² . In particular, when a Sargent bath is used, preferably at a current density of 45 to 70 A / dm ² , more preferably 50 to 65 A / dm ² , still more preferably 50 to 65 A / dm ² , particularly preferably 55 to 65 A / dm ² , It is preferably performed for 30 minutes or more, more preferably 40 to 120 minutes, further preferably 50 to 90 minutes, particularly preferably 60 to 80 minutes. As the current density is higher and the chrome plating time is longer, the amount of coulomb increases, resulting in an increase in the thickness of the release layer 13. As described above, when the thickness of the release layer 13 increases, the aperture ratio tends to increase as described above. In particular, the aperture ratio increases at a high current density of around 60 A / dm ² . A preferable bath temperature in electrolytic chrome plating is 45 to 65 ° C, more preferably 45 to 60 ° C.

A stable chromium plating bath typically contains a small amount of trivalent chromium, and the amount is about 2 to 6 g / L. Further, a catalyst such as organic sulfonic acid may be added to the hard chromium plating bath. The concentration of chromic anhydride can be controlled by the Baume degree. Furthermore, since impurities such as iron, copper, and chloride ions affect the state of plating, care must be taken in managing the upper limit of the amount of impurities dissolved. As an anode used for chromium plating, a titanium-coated lead oxide or Pb—Sn alloy can be preferably used. As a typical commercial product of such an anode, a Ti—Pb electrode (Sn of SnF) is used. : 5%) and Exelod LD manufactured by Nippon Carlit.

Prior to the formation of the porous metal foil by electroplating, the release layer 13 is preferably subjected to washing, drying, and heat treatment. Washing may be performed with an aqueous solvent such as water, or with an organic solvent such as acetone. Drying may be performed by either natural drying or heat drying. The heat treatment is preferably performed at 80 to 180 ° C. for 2 to 16 hours, more preferably at 130 to 170 ° C. for 4 to 8 hours. This heat treatment is preferably performed in an oxygen-containing atmosphere such as an air atmosphere. By this heat treatment, the surface of the release layer 13 is oxidized, and Cr ₂ O ₃ is formed as a passive state. This has an advantage that the porous metal foil 10 can be easily peeled off.

(3) Formation of porous metal foil by electrolytic plating Next, the release layer 13 is electroplated with a metal that can be preferentially deposited on the crack 13a, and the innumerable metal particles 11 are grown along the crack 13a. Thereby, a porous metal foil 10 having a two-dimensional network structure composed of metal fibers and having an open area ratio exceeding 80% is formed. As described above, the release layer 13 has the crack 13a having the property of being easily plated and the crack-free surface portion 13b having the property of being difficult to be plated. The reason why plating with the crack 13a is facilitated is that the current is more likely to flow in the portion where the crack 13a is present than in the portion 13b where the crack 13a is not present, so that nucleation and growth occur preferentially in the crack 13a. The metal that can be preferentially deposited in the crack 13a preferably comprises at least one selected from the group consisting of copper, gold, silver, nickel, cobalt, tin, and zinc, and more preferably copper, silver, And at least one selected from the group consisting of gold, and more preferably copper.

The conditions for the electroplating of the metal that can be preferentially deposited in cracks are in accordance with the known conditions using various known metal plating baths, except that the current density and time are set so as to give an opening ratio exceeding 80%. Just do it. Such electrolytic plating is preferably performed at a current density of 0.5 to 10 A / dm ² , more preferably 1 to 8 A / dm ² , even more preferably 2 to 6 A / dm ² , preferably 1 to 500 seconds, and more. It is preferably performed for 3 to 150 seconds, more preferably 5 to 75 seconds. Thus, it becomes easier to realize a high aperture ratio by performing electroplating for a short time at a considerably low current density. The bath temperature is preferably 10 to 60 ° C, more preferably 15 to 55 ° C, and further preferably 20 to 50 ° C.

In particular, when the metal capable of preferentially precipitating in cracks is copper, the electrolytic copper plating is preferably performed at a current density of 1 to 5 A / dm ² for 2 to 250 seconds, more preferably 1.5 to 4 2-170 seconds at a current density of .5A / dm ^2, still preferably performed 2.5 to 120 seconds at a current density of ² ~ 4A / dm 2. Thus, it becomes easier to realize a high aperture ratio by performing electroplating for a short time at a considerably low current density. Electrolytic copper plating is preferably performed using a copper sulfate plating bath, and the preferred composition of the copper sulfate plating bath is a copper sulfate pentahydrate concentration: 150 to 320 g / L, and a sulfuric acid concentration: 15 to 200 g / L. . The preferred bath temperature for copper sulfate plating is 15 to 55 ° C., more preferably 20 to 50 ° C., and further preferably 25 to 45 ° C.

An additive may be appropriately added to the plating solution to improve the characteristics of the metal foil. For example, in the case of copper foil, preferred examples of such additives include sulfur-containing compounds such as glue, gelatin, chlorine and thiourea, and synthetic additives such as polyethylene glycol. By using these preferable additives, the mechanical properties and surface state of the metal foil can be controlled. The concentration of the additive is not limited, but is usually 1 to 300 ppm.

(4) Peeling of porous metal foil If necessary, the porous metal foil can be peeled off from the conductive substrate having the release layer to obtain a single porous metal foil. After peeling, it may be transferred to another substrate such as a film with an adhesive layer, or peeling itself may be performed by transfer to another substrate. For example, the peeling of the porous metal foil can be performed with an adhesive tape or by immersing in an etching solution, and various methods can be adopted. However, this peeling step is not essential, and the substrate may be handled as a porous metal foil product with the substrate attached via the peeling layer, and may be peeled off for the first time during use.

The present invention will be described more specifically with reference to the following examples.

Example 1 Production of Porous Metal Foil A stainless steel plate having a thickness of 0.5 μm was prepared as a conductive substrate. Chromium plating was performed on the stainless steel plate as a release layer by the following procedure. First, after dipping in acetone (99.0%, manufactured by Wako Pure Chemical Industries) for 10 seconds, the surface is cleaned by washing with pure water and drying. Next, the stainless steel plate foil was immersed in a Sargent bath in which 2.5 g / L sulfuric acid and 250 g / L chromic acid were dissolved, bath temperature: 50 ° C., current density: 60 A / dm ² , anode: Pb, Chromium plating was performed for 72 minutes under the condition of cathode: stainless steel plate. The stainless steel plate on which the chromium plating layer was formed was washed with acetone and then dried. When the thickness of the obtained chrome plating was measured by XRF (fluorescence X-ray analysis), it was about 15 μm, and numerous cracks generated by plating stress were confirmed on the surface of the chrome plating. The dried chrome plating layer was heat-treated at 150 ° C. for 5 hours in an air atmosphere.

Copper sulfate plating was performed on the chromium plating in which the cracks occurred. In this copper sulfate plating, a stainless steel plate coated with chromium was immersed in a copper sulfate plating bath in which 25 g / L of sulfuric acid and 200 g / L of copper sulfate pentahydrate were dissolved, and the current density was 3 A / dm ² . Bath temperature: 25 ° C., anode: Cu, cathode: chromium plating layer, 75 seconds (sample 1), 30 seconds (sample 2), 15 seconds (sample 3), 7 seconds (sample 4) or 3 seconds (sample) 5) I went. At this time, since the current flows more easily in the crack portion than in the outermost surface of the chromium plating, the copper particles grew from the crack. As a result, a two-dimensional network structure composed of copper fibers was formed on the chrome plating as a porous metal foil.

Example 2 Observation of Porous Metal Foil The porous metal foils of Samples 1 and 3 obtained in Example 1 were observed from directly above with a field emission scanning electron microscope (FE-SEM). The image shown in 5A was obtained. Further, when the metal fibers of the porous metal foils of Samples 1 and 3 were observed with a field emission scanning electron microscope (FE-SEM), the images shown in FIGS. 4B and 5B were obtained, respectively. As is clear from these figures, bead-like or worm-like (caterpillar-like) irregularities due to the spherical portions of the metal particles were observed on the growth surface.

Furthermore, when the cross section of the metal fiber in the porous metal foils of

Samples

2, 3 and 4 was processed using a focused ion beam processing apparatus (FIB) and then observed using a scanning ion microscope (SIM), FIG. Images shown in 6B and 6C were obtained. As shown in FIGS. 6A to 6C, the cross-sectional structure of the metal fiber is precipitated radially starting from the crack, and the cross-sectional shape of the metal fiber is a semicircular shape including a spherical portion and a planar bottom surface. Observed. 6A to 6C, it seems that the cross section of the metal fiber has a two-layer structure, which is because the metal fiber is previously coated with carbon in order to clearly observe the processed surface. It was about 0.50 when the ratio with respect to the wire diameter D of the largest cross-section height H in a metal fiber cross section was computed.

Example 3 : Peeling of porous metal foil (1) Peeling with adhesive tape High tackiness on the surface of samples 1 to 5 (formed on a conductive substrate through a peeling layer) obtained in Example 1 A pressure-sensitive adhesive tape (manufactured by Nitoms Co., Ltd., super-transparent double-sided pressure-sensitive adhesive sheet, product number T: 284) was attached, and the pressure-sensitive adhesive tape was peeled off. As a result, as shown in FIG. 7, the porous metal foil was peeled off in a form transferred to the adhesive tape. The peeled porous metal foil was attached to a glass plate.

(2) Stripping with Cr etching solution Samples 1 to 5 (formed on a conductive substrate through a peeling layer) obtained in Example 1 were treated with Cr etching solution (Mekku Co., Ltd. -1925) at 40-50 ° C. for several minutes. As a result, as shown in FIG. 8, the porous metal foil was peeled off in an isolated or self-supporting form.

Example 4 : Measurement of aperture ratio, wire diameter and thickness The aperture ratio, wire diameter and thickness of the porous metal foil obtained in Example 1 were measured as follows.
(Measurement method of aperture ratio)
Using an electron microscope, an SEM photograph of the porous metal foil at a magnification of 100 was taken so that the observation area was 1.137 mm ² . Next, the image processing software: Image-J was used to identify the metal fiber portion and the opening portion, and the ratio of the opening portion area to the total observation area was calculated and used as the opening ratio. More specifically, a) Import SEM image into image analysis software Image-J, b) Click [Image>Adjust> Threshold Setting] in the tab and the copper fiber part and chrome plating part displayed in the imported image C) Click [Image Analysis> Execute Image Analysis] on the tab to calculate the ratio of colors on the image, and d) Set the ratio of colors applied to the chrome plating part. Obtained as the aperture ratio. That is, the aperture ratio in this example was calculated as the ratio of the area where the chrome plating portion was visible in the SEM observation image.
(Measurement method of wire diameter)
The wire diameter of the metal fiber was measured by observation with an electron microscope at a magnification of 5,000 to 10,000 from directly above the porous metal foil.
(Thickness measurement method)
The thickness of the porous metal foil was measured using a surface shape measuring device Dektak 150 (manufactured by ULVAC, Inc.).

As a result of the measurement, the aperture ratio, wire diameter, and thickness shown in Table 1 were obtained. FIG. 9 shows the relationship between the aperture ratio and the copper plating time, and FIG. 10 shows the relationship between the wire diameter and the copper plating time. From these results, it can be seen that a porous metal foil having a small wire diameter and an extremely high aperture ratio can be obtained by providing a thick chromium plating and performing copper sulfate plating for a short time at a low current density.

Example 5 : Relationship between aperture ratio and transmittance In order to investigate the relationship between aperture ratio and transmittance, various conditions such as chromium plating and copper sulfate plating were appropriately changed in Example 1 so that the aperture ratio was 71.0%, 46 0.0% and 13.2% porous copper foils were produced. When the transmittance in the visible light region of the obtained porous copper foil was measured with an absorptiometer, the result shown in FIG. 11 was obtained. Moreover, it was as Table 2 when the numerical value of the aperture ratio of each copper foil and the transmittance | permeability in wavelength 550nm was written together.

As can be seen from FIG. 11 and Table 2, it was confirmed that the aperture ratio of the porous metal foil had a high correlation with the transmittance in the visible light region, and that there was no specific absorption peak in the visible light region. . Therefore, the porous metal foil of the present invention having an aperture ratio of 80%, preferably more than 85%, has a very high light transmittance in the visible light region (approximately more than 80%, preferably more than 85%). However, it can be seen that it is extremely suitable for the use of the transparent conductive film.

Claims

A porous metal foil having a two-dimensional network structure composed of irregularly stretched metal fibers, wherein the metal fibers have a substantially semi-circular or substantially semi-elliptical cross-sectional shape, and the porous metal foil A porous metal foil having an opening ratio of more than 80%.
The porous metal foil according to claim 1, wherein the aperture ratio is 85% or more.
The porous metal foil according to claim 1 or 2, wherein the metal fiber has a wire diameter of 14 µm or less.
The porous metal foil according to claim 1 or 2, wherein the metal fiber has a wire diameter of 4 µm or less.
The porous metal foil according to any one of claims 1 to 4, wherein the metal fiber is a branched fiber.
The porous metal foil according to any one of claims 1 to 5, wherein the metal fiber is formed by connecting innumerable metal particles.
The metal particles have a hemispherical shape having a spherical portion and a bottom portion, the bottom portions of all the metal particles are located on the same base surface, and the spherical portions of all the metal particles are based on the base surface. The porous metal foil according to claim 6, which is located on the same side.
The porous metal foil according to any one of claims 1 to 7, wherein the porous metal foil has a thickness of 1 to 7 µm.
The porous metal foil according to any one of claims 1 to 8, wherein the two-dimensional network structure has an irregular shape caused by a crack.
The porous metal foil according to any one of claims 1 to 9, wherein the metal fiber comprises at least one selected from the group consisting of copper, gold, silver, nickel, cobalt, tin, and zinc. .
The porous metal foil according to any one of claims 1 to 10, wherein the metal fiber has a substantially semi-elliptical cross-sectional shape, and an average ratio of a maximum cross-sectional height to a wire diameter exceeds 0.50.
A method for producing a porous metal foil,
Forming a release layer made of chromium, a chromium alloy and / or a chromium oxide by performing electrolytic chromium plating on the conductive substrate, and generating a crack in the release layer by the stress of the release layer itself;
The release layer is electroplated with a metal that can be preferentially deposited on the cracks, and an infinite number of metal particles are grown along the cracks, thereby comprising a two-dimensional network structure composed of metal fibers and 80 Forming a porous metal foil having an open area ratio greater than%,
A manufacturing method comprising:
The method according to claim 12, wherein the release layer is cleaned, dried, and heat-treated before electrolytic plating of metal that can be preferentially deposited on the cracks.
The method according to claim 12 or 13, further comprising a step of peeling the porous metal foil from the release layer.
The method according to any one of claims 12 to 14, wherein the electrolytic chromium plating is performed using a surge bath.
The method according to any one of claims 12 to 15, wherein the electrolytic chromium plating is performed at a current density of 30 to 100 A / dm 2 for 20 minutes or more.
The method according to any one of claims 12 to 15, wherein the electrolytic chromium plating is performed at a current density of 45 to 70 A / dm 2 for 30 minutes or more.
The method according to any one of claims 12 to 17, wherein the release layer has a thickness of 4 to 120 µm.
The metal capable of preferentially precipitating in the crack comprises at least one selected from the group consisting of copper, gold, silver, nickel, cobalt, tin, and zinc. The method described in 1.
The method according to any one of claims 12 to 19, wherein the electrolytic plating of the metal capable of preferentially precipitating in the crack is performed at a current density of 0.5 to 10 A / dm 2 for 1 to 500 seconds.
The metal that can be preferentially deposited in the crack is copper, and the electrolytic plating of copper is performed at a current density of 1 to 5 A / dm 2 for 2 to 250 seconds. the method of.