Metal nonwoven fabric and electrode comprising same

Abstract

This metal nonwoven fabric comprises a metal fiber and is liquid permeable. An average fiber diameter of the metal fiber is between 20 nm and 10 µm both inclusive. In the metal nonwoven fabric, a distribution peak top diameter of voids measured according to the mercury porosimetry method is 30 µm or less. In the metal nonwoven fabric, a distribution peak top diameter of voids measured according to the mercury porosimetry method is preferably 0.01 µm or greater. Where the length of a metal crystal constituting the metal fiber along the extending direction of the metal fiber is X, and the length along a direction orthogonal to said direction is Y, an arithmetic mean value of the value of X / Y, which is the ratio of X to Y, is preferably 4 or less at three points in boundary regions that are obtained by dividing the length along the extending direction of the metal fiber into four equal parts.

Description

Metallic nonwoven fabric and electrode made using the same

[0001] The present invention relates to a metal nonwoven fabric and an electrode made using the same.

[0002] Fine metal fibers with diameters of approximately several tens of nanometers to several tens of micrometers are known. Due to their fine diameters and high aspect ratios, such metal fibers are expected to exhibit physical and chemical properties (e.g., electrical conductivity, thermal conductivity, luminescence properties, catalytic activity, etc.) that are not found in conventional materials. Known conventional technologies relating to the use of metal fibers include those described in Patent Document 1 and Non-Patent Document 1.

[0003] Patent Document 1 describes a porous electrode with Cu nanowires produced by reacting an aqueous solution of sodium hydroxide, an aqueous solution of copper nitrate, an aqueous solution of ethylenediamine, and an aqueous solution of α-D-glucose and heating the reaction mixture. The document also describes that this porous electrode has a larger surface area than carbon paper and exhibits low electrical resistivity.

[0004] Non-Patent Document 1 describes the use of an electrode made of porous felt using nickel microfibers for alkaline water electrolysis. The document also describes that the high surface area of ​​the electrode makes it easier for oxygen bubbles generated by alkaline water electrolysis to escape from the electrode, improving the hydrogen production rate at the counter electrode.

[0005] US2019 / 0305322A1

[0006] F. Yang, et al., Adv. Energy Mater, 2020, 10, 20011174.

[0007] The techniques described in Patent Document 1 and Non-Patent Document 1 utilize the fine diameter of metal fibers, thereby providing electrodes with large surface areas. When using metal fibers, liquid permeability is sometimes required so that physical and chemical properties can be exerted, but Patent Document 1 and Non-Patent Document 1 make no mention of this point. The present inventors therefore conducted extensive research into methods for utilizing metal fibers and found that when a nonwoven fabric is produced using metal fibers so as to have predetermined voids, liquids can efficiently permeate the nonwoven fabric, and reactants in the liquid that have soaked into the voids can efficiently reach the surfaces of the metal fibers, making it easier to utilize the desired physical and chemical properties.

[0008] That is, the present invention provides a liquid-permeable metal nonwoven fabric comprising metal fibers, wherein the metal fibers have an average fiber diameter of 20 nm or more and 10 μm or less, and the void distribution peak top diameter measured by mercury porosimetry is 30 μm or less.

[0009] The present invention will be described below based on preferred embodiments. The present invention relates to a metal nonwoven fabric. The metal nonwoven fabric of the present invention comprises metal fibers made of metal. In the following description, the term "metal fiber" may refer to an individual fiber or an aggregate of multiple fibers, depending on the context.

[0010] A metal nonwoven fabric is a sheet-like fiber assembly primarily composed of metal fibers. In this specification, "primarily composed of metal fibers" refers to a state in which the metal nonwoven fabric contains 50% or more by mass of metal fibers. A metal nonwoven fabric may contain other constituent materials in addition to metal fibers, such as organic fibers, carbon fibers, or oxide fibers. It may also contain substances in a form other than fibrous. Therefore, a metal nonwoven fabric may be a sheet-like fiber assembly containing multiple constituent materials, including metal fibers. However, it is desirable for a metal nonwoven fabric to consist essentially of metal fibers in order to maximize the inherent advantages of the metal nonwoven fabric. In this specification, the phrase "consisting essentially of metal fibers" excludes the intentional addition of components other than metal fibers to the nonwoven fabric, and allows for trace components that are inevitably mixed in during the manufacturing process of the metal nonwoven fabric.

[0011] The metal nonwoven fabric may be composed of a single type of metal fiber, or may contain multiple types of metal fibers, including first metal fibers made of a first metal type and second metal fibers made of a second metal type.

[0012] A metal nonwoven fabric maintains its fabric form by randomly depositing the metal fibers that make up the fabric and bonding them together through entanglement and / or fusion. Due to the random deposition of the metal fibers, the metal nonwoven fabric forms multiple voids between the fibers, which are continuous in the thickness direction of the fabric and in other directions. This allows the metal nonwoven fabric to have liquid permeability in the thickness direction and other directions. A metal nonwoven fabric may have a single-layer structure or a laminated structure in which multiple layers are laminated together. When the metal nonwoven fabric has a laminated structure, the layers may be identical or different. Here, "identical" means that the metal fibers that make up the metal nonwoven fabric are the same. Such a metal nonwoven fabric can be suitably manufactured, for example, by the manufacturing method described below.

[0013] It is preferable that the metal nonwoven fabric has liquid permeability as a whole. The liquid permeability of the metal nonwoven fabric is mainly manifested by the fine voids in the nonwoven fabric. The degree of liquid permeability in the metal nonwoven fabric is appropriately adjusted depending on the application of the metal nonwoven fabric. As will be described later, the liquid permeability of the metal nonwoven fabric can be improved by adjusting, for example, the average fiber diameter of the metal fibers in the metal nonwoven fabric or the size of the voids within the metal nonwoven fabric.

[0014] Liquid permeability occurs due to capillary action and is theoretically determined by the pore size, the physical and chemical state of the surface of the material constituting the metal nonwoven fabric, and the relationship between the surface tension of the metal nonwoven fabric and the surface tension of the liquid, which is determined thereby. Here, in the present invention, liquid permeability is defined as a characteristic value that can be measured by the following method. First, a sample 25 mm long x 5 mm wide is cut out from a dry metal nonwoven fabric. Next, the thickness and mass of the sample are measured. The sample is suspended over a glass container (circular with a bottom diameter of 15 mm) containing water, and the tip of the sample is immersed 5 mm into the water in the container. Water then penetrates and is absorbed into the sample. After 15 minutes, the sample is removed from the container and the mass of the water remaining in the glass container is measured. The volume of the sample is determined based on the dimensions of the outer contour of the sample before water absorption, and the average amount of water absorbed per volume is calculated; this value is 0.4 mL / cm. ３ In this specification, a material that satisfies the above criteria is deemed to have "liquid permeability." A specific method for measuring the volume will be described later.

[0015] As long as the metal nonwoven fabric as a whole is liquid permeable, it may be a three-layer metal nonwoven fabric, for example, with one or more layers of mesh or perforated foil (hereinafter also referred to as "mesh, etc.") disposed between two layers of metal nonwoven fabric. Alternatively, it may be a three-layer or more metal nonwoven fabric, with one layer of metal nonwoven fabric disposed between two layers of mesh, etc. This can improve the strength of the metal nonwoven fabric. The average mesh opening (JIS Z8801) or the average pore size of the perforated foil is preferably 1000 μm or less, and may be 900 μm or less. The mesh, etc. may be made of metal or nonmetal. When the mesh, etc. is made of metal, the type of metal constituting the mesh, etc. and the type of metal constituting the metal nonwoven fabric may be the same or different.

[0016] From the viewpoint of improving the liquid permeability of the metal nonwoven fabric and allowing the reactant in the permeated liquid to reach the metal fibers that make up the metal nonwoven fabric in a short period of time, the average fiber diameter of the metal fibers is preferably 20 nm or more and 10 μm or less, more preferably 20 nm or more and 6 μm or less, even more preferably 20 nm or more and 5 μm or less, still more preferably 30 nm or more and 3 μm or less, even more preferably 90 nm or more and 3 μm or less, and particularly preferably 120 nm or more and 2.5 μm or less.

[0017] From the viewpoint of improving the liquid permeability of the metal nonwoven fabric, maintaining the mechanical strength, and easily producing the metal nonwoven fabric, the average length of the metal fibers is preferably 0.5 μm or more and 5000 μm or less, more preferably 0.5 μm or more and 2000 μm or less, even more preferably 1 μm or more and 500 μm or less, still more preferably 3 μm or more and 200 μm or less, even more preferably 3 μm or more and 150 μm or less, and particularly preferably 3 μm or more and 90 μm or less.

[0018] The average fiber diameter and average length of the metal fibers in the metal nonwoven fabric may be the same or different on one side and the other side of the metal nonwoven fabric. Alternatively, when viewed along the thickness direction of the metal nonwoven fabric, the average fiber diameter and average length of the metal fibers may change stepwise, continuously, or a combination thereof. Such a metal nonwoven fabric may be formed, for example, by using multiple types of metal fibers.

[0019] As described above, it is preferable that the metal fibers in the metal nonwoven fabric are very thin and long. By having the metal fibers have such an average fiber diameter and average length, the liquid permeability of the metal nonwoven fabric is improved and reactants in the permeated liquid can reach the surface of the metal fibers that make up the metal nonwoven fabric in a short period of time. In addition, it becomes easier to control these characteristic values ​​of the metal fibers and to improve their handleability.

[0020] From the viewpoint of improving the liquid permeability of the metal nonwoven fabric and allowing the reactant in the permeated liquid to reach the metal fibers that make up the metal nonwoven fabric in a short period of time, the aspect ratio of the metal fibers (length of metal fiber [m] / fiber diameter of metal fiber [m]) is preferably 5 or more and 5,000 or less, more preferably 13 or more and 3,000 or less, even more preferably 20 or more and 3,000 or less, even more preferably 20 or more and 1,500 or less, and even more preferably 20 or more and 800 or less.

[0021] The average fiber diameter of metal fibers in a metal nonwoven fabric can be measured by the following method. Specifically, the metal nonwoven fabric is first observed using a scanning electron microscope (hereinafter also referred to as "SEM"). The metal nonwoven fabric is observed at a magnification that makes it easy to observe the fiber diameters of the metal fibers that make up the metal nonwoven fabric, specifically at a magnification between 1,000x and 10,000x. From the SEM image of the obtained metal nonwoven fabric, 10 metal fibers are randomly selected, excluding those that are entangled with other metal fibers and cannot be measured individually, and the fiber diameter of each metal fiber is calculated. The average fiber diameter of the metal fibers is obtained by arithmetically averaging these.

[0022] The average length of metal fibers in a metal nonwoven fabric can be measured by the following method. Specifically, the metal nonwoven fabric is first observed using an SEM. The metal nonwoven fabric is observed at a magnification that makes it easy to observe the lengths of the metal fibers that make up the metal nonwoven fabric, specifically at a magnification between 200x and 2000x. From the SEM image of the obtained metal nonwoven fabric, 20 metal fibers are randomly selected, excluding those that are entangled with other metal fibers and cannot be measured individually. If the metal fibers extend beyond the screen of a single SEM image, the length of each metal fiber is calculated by panoramic stitching of multiple consecutive SEM images. The arithmetic mean is then calculated to obtain the average length of the metal fibers.

[0023] The metal fibers in a metal nonwoven fabric may have a substantially uniform average fiber diameter throughout their entire length, or may have a non-uniform average fiber diameter in the shape of a bead. It is preferable that at least one end of the metal fibers is tapered. A "tapered shape" refers to a shape in which, when observing the end region of a metal fiber, the thickness gradually decreases toward the tip. Having at least one end of the metal fibers tapered increases the contact area between the metal fibers, which is advantageous from the perspective of exhibiting electrical conductivity and thermal conductivity and maintaining mechanical strength. Furthermore, metal fibers with at least one tapered end reduce the unevenness of the contact surface compared to metal fibers with a uniform average fiber diameter, thereby suppressing variation in the size of voids in the metal nonwoven fabric.

[0024] In order to make this advantage even more pronounced, the angle of the tip of the tapered shape is preferably 90 degrees or less, more preferably 80 degrees or less, even more preferably 70 degrees or less, and may be 60 degrees or less, 50 degrees or less, or 45 degrees or less.

[0025] The angle of the tapered tip is measured using the following procedure. First, as described above, the fiber diameter of the metal fiber is measured using an SEM at a magnification between 1,000x and 10,000x. Next, an arc having a diameter equal to the fiber diameter of the metal fiber is drawn with the end tip of the metal fiber as its center, and two intersections between the arc and the metal fiber are obtained. The angle between the two intersections and the end tip of the metal fiber is measured as the tip angle. Note that if the cross section of the end of the metal fiber is linear or approximately linear, the center of the end is taken as the tip. Furthermore, if the cross section of the end of the metal fiber is linear or approximately linear and the cross-sectional length exceeds half the fiber diameter of the metal fiber, the metal fiber is excluded from the measurement. This measurement is performed on 10 or more metal fibers, and the arithmetic average value is taken as the angle of the tapered tip.

[0026] The metal fibers in metal nonwoven fabrics typically have the shape of fibers extending in one direction. These metal fibers may or may not have a main chain extending in one direction and a branched structure in which the main chain branches off midway. From the viewpoint of controlling the size of voids in the metal nonwoven fabric and improving liquid permeability, it is preferable that the metal fibers have an unbranched structure having only a main chain. On the other hand, from the viewpoint of making the metal nonwoven fabric bulky and further increasing the contact area with liquid, it is preferable that the metal fibers have one or more branched portions. Furthermore, the metal nonwoven fabric may be formed using only metal fibers having an unbranched structure having only a main chain, or may be formed using only metal fibers having one or more branched portions, or may be formed using a combination thereof.

[0027] There are no particular limitations on the type of metal constituting the metal fibers in the metal nonwoven fabric, and various metals can be used. Considering the balance between high electrode conductivity and ease of industrial use, examples of suitable metals include copper, silver, gold, nickel, lead, palladium, platinum, cobalt, tin, iron, zinc, or bismuth, or alloys containing these metals. The metal constituting the metal fibers is preferably copper, nickel, tin, or zinc, or an alloy containing these metals. Alternatively, the fibers may be formed from a mixture of crystals of multiple metals or alloys. Among these, fibers made of copper or a copper alloy are particularly preferred. Note that "made of copper or a copper alloy" refers to a metal fiber in which the copper content is 60% by mass or more. An example of a mixture of crystals of multiple metals or alloys is a series of crystals of different metals, such as Cu crystal-Ni crystal-Cu crystal-Ni crystal.

[0028] The surfaces of metal fibers in metal nonwoven fabrics may be coated with a substance other than metal. Here, "coating" includes both chemical bonding of the metal and the nonmetallic substance constituting the metal fiber and physical adsorption of the metal and the nonmetallic substance constituting the metal fiber. Metal fibers may be coated in either one of these states or both of these states. Examples of substances other than metals that may be coated on metal fibers in metal nonwoven fabrics include oxides, sulfides, organic substances, carbon materials, semiconductor materials, ferroelectric materials, magnetic materials, MOFs (metal-organic frameworks), and PCPs (porous coordination polymers). Among these, when metal nonwoven fabrics are used as electrode catalysts, it is preferable to coat the metal fibers with an oxide, organic substance, carbon material, or semiconductor material. The method for coating the metal fibers with other substances is not particularly limited. Examples of such methods include a method in which metal fibers are deposited by the method described below, and then electroplated in an electrolytic bath containing the substance used for coating, cationic electrodeposition coating, anionic electrodeposition coating, surface oxidation treatment (electrically or chemically oxidizing the surface to cause a reaction with chemical substances in the solution to form a film), displacement plating, electroless plating, a method in which a catalyst used in these plating methods is applied to metal fibers, and then the desired substance is plated, liquid phase deposition, electrophoresis, a method utilizing a surface potential difference, a sol-gel method, a gel-sol method, a polyol method, a spray method, a cold spray method, a spray-dry method, a dipping method, a vapor deposition method, a sputtering method, a CVD method, a thermal decomposition method, a plasma film formation method, a dipping method, an atomic layer deposition method (ALD method), a coating method, an ink method, a fine particle coating method, or a dry coating method.

[0029] The metal fibers in the metal nonwoven fabric may comprise a core made of a metal and a shell made of a metal other than the metal core and disposed on the surface of the core. In other words, the metal fibers in the metal nonwoven fabric may be formed by laminating different types of metals. The core constitutes the main portion of the metal fiber. The shell may be disposed over the entire surface of the core. Alternatively, the shell may be present so as to cover a portion of the surface of the core. The boundary between the core and shell may be clear, or there may be an unclear portion at the boundary within a range in which the core and shell can be distinguished. Examples of metals constituting the core include copper, silver, gold, nickel, lead, palladium, platinum, cobalt, tin, iron, zinc, or bismuth, or alloys containing these metals. The metal constituting the shell is not particularly limited as long as it is a metal different from the metal constituting the core, and examples include copper, silver, nickel, tin, zinc, lead, iron, cobalt, platinum, gold, and palladium. The method for producing metal fibers consisting of a core portion and a shell portion is not particularly limited. Examples of the method include a method in which metal fibers are deposited by the method described below and then electroplated in an electrolytic bath containing a substance used for coating, a displacement plating method, an electroless plating method, a method in which a catalyst used for these plating methods is attached to metal fibers and then the target substance is plated, an electrophoresis method, a method utilizing a surface potential difference, a polyol method, a spray method, a cold spray method, a spray-drying method, a sputtering method, a CVD method, a pyrolysis method, a plasma film formation method, an atomic layer deposition method (ALD method), an ink method, a fine particle coating method, or a dry coating method.

[0030] The metal fibers in metal nonwoven fabrics preferably have a polycrystalline structure in which multiple crystals are connected along the direction of extension of the metal fibers (hereinafter also referred to as the "longitudinal direction"). Due to their characteristic crystalline structure, such metal fibers lower the temperature at which metal fibers fuse together, allowing the metal fibers to fuse at lower temperatures, thereby enabling metal nonwoven fabrics to be formed under milder conditions at lower temperatures. Furthermore, since the metal fibers are more likely to assume a curved shape than a linear shape, the mechanical strength of the metal nonwoven fabric can be increased. Furthermore, since the crystal faces that are susceptible to oxidation are not preferentially exposed on the side surfaces of the metal fibers, oxidation of the side surfaces of the metal fibers is suppressed. Furthermore, as mentioned above, the ends of the metal fibers can be tapered, which increases the contact area between the metal fibers and facilitates the development of electrical and thermal conductivity. Note that the "extension direction" of the metal fibers refers to the tangential direction when the metal fibers have curved portions.

[0031] To describe the crystal structure of metal fibers in detail, when the length of the metal crystals constituting the metal fibers along the longitudinal direction of the metal fiber is defined as X and the length along the direction perpendicular to the longitudinal direction (hereinafter also referred to as the "width direction") is defined as Y, the ratio of X to Y, i.e., X / Y, is preferably 4 or less. Thus, it is preferable that the metal crystals constituting the metal fibers have a substantially isotropic shape with no significant difference between the longitudinal length and the width length. Since the average fiber diameter of the metal fibers is 20 nm or more and 10 μm or less as described above, it is understood that the metal crystals constituting the metal fibers are generally fine. Due to the structure of the metal crystals constituting the metal fibers in metal nonwoven fabrics, as described above, the temperature at which the metal fibers fuse together is lowered, allowing the metal fibers to fuse at a lower temperature, thereby enabling the metal nonwoven fabric to be formed under milder conditions at a lower temperature. Furthermore, since metal fibers are more likely to assume a curved shape than a linear shape, the mechanical strength of the metal nonwoven fabric can be increased. Furthermore, the crystal planes that are easily oxidized on the side surfaces of the metal fibers are not preferentially exposed, thereby suppressing oxidation of the side surfaces of the metal fibers. Furthermore, since the ends of the metal fibers can be tapered, the contact area between the metal fibers is increased, making it easier for electrical conductivity and thermal conductivity to be exhibited. To further enhance this advantage, the X / Y value is preferably 3 or less, and even more preferably 2.5 or less. The above-mentioned X / Y value is calculated in the following manner. First, crystals at three locations in the boundary region when a single metal fiber constituting the metal nonwoven fabric is divided into four equal parts along the longitudinal direction are measured. Similar measurements are repeated for two or more metal fibers. The value is calculated by calculating the average value for a single metal fiber based on the X / Y values ​​at each location on the metal fiber (not calculating X / Y from the average X and Y values, but averaging the X / Y values ​​at each location), and then calculating the arithmetic mean value from the X / Y values ​​read for each of the multiple metal fibers. The arithmetic mean value is rounded to the nearest tenth.Although the term "boundary region when the length is divided into four equal parts along the longitudinal direction" is used, if measurement at the boundary region is not possible for measurement reasons, measurement will be made at a region close to the boundary region (for example, a region within 10% of the length of the metal fiber on each side of the boundary region).

[0032] As used herein, "crystal" refers to a crystal grain, the size of which can be measured by electron backscatter diffraction (hereinafter also referred to as "EBSD") method. It should be noted that the concept of crystal grain is different from the crystallite size determined from an XRD pattern. When the crystal referred to herein is a twin crystal, the crystals constituting the twin crystal are defined as different crystals. Furthermore, even if it is not possible to confirm whether a crystal is a twin crystal or not, if even a part of the line indicating a grain boundary is observed by EBSD, it is defined as a different crystal, and the value of X / Y is determined for each crystal.

[0033] When the value of X / Y is 4 or less, there are no restrictions on the values ​​of X and Y, but from the viewpoint of being able to lower the fusion temperature of the metal fibers, the value of X itself is preferably 10 μm or less, more preferably 5 nm or more and 2 μm or less, and even more preferably 10 nm or more and 500 nm or less. From the same viewpoint, the value of Y itself is preferably 3 μm or less, more preferably 5 nm or more and 1 μm or less, and even more preferably 10 nm or more and 400 nm or less.

[0034] On the other hand, when the X / Y value exceeds 4, the metal fiber can be characterized solely by the above-mentioned Y value. That is, the Y value is preferably 10 nm or less. A Y value of 10 nm or less means that the metal fiber is thin, with a width equivalent to 100 or fewer metal atoms. This is the same as the design concept of an X / Y value of 4 or less, and is equivalent to the crystals being fine. As a result, such metal fibers can lower the temperature at which the metal fibers fuse in the metal nonwoven fabric of the present invention. Note that as long as Y is 10 nm or less, the X / Y value is not important. The above-mentioned Y value is calculated using the following method. First, the Y values ​​are measured for crystals at three locations in the boundary region when a single metal fiber is divided into four equal parts along the longitudinal direction. Similarly, measurements are repeated for two or more metal fibers, and the arithmetic average of these values ​​is calculated. The arithmetic average value is rounded to the nearest tenth. Although the term "boundary region when the length is divided into four equal parts along the longitudinal direction" is used, if measurement at the boundary region is not possible for measurement reasons, measurement will be made at a region close to the boundary region (for example, a region within 10% of the length of the metal fiber on each side of the boundary region).

[0035] Metal fibers in metal nonwoven fabrics are also characterized by the orientation of the metal crystals that make them up. Specifically, when focusing on the crystals present in three boundary regions when the metal fiber is divided into four equal parts along its extension direction, the proportion of crystal grains with the

[110] orientation evaluated by electron diffraction or electron beam scattering diffraction (EBSD) using a transmission electron microscope (hereinafter also referred to as "TEM") within a range of ±30° along the extension direction of the metal fiber is preferably 50% or less, more preferably 45% or less, and even more preferably 40% or less. Satisfying this relationship means that the

[110] orientation of the crystals is not preferentially oriented in the longitudinal direction of the metal fiber. The proportion of crystal grains with the

[110] orientation is calculated by randomly extracting two or more metal fibers from a metal nonwoven fabric, drawing boundary lines that divide each metal fiber into four equal parts along the longitudinal direction, and measuring the three boundary regions of the boundary lines. When evaluated by TEM electron diffraction, this is the percentage of crystal grains with the

[110] orientation measured at a total of six or more midpoints of the boundary lines per boundary region (e.g., six midpoints if two metal fibers are extracted, and 15 midpoints if five metal fibers are extracted). When evaluated by EBSD, measurements are made at a total of 18 or more midpoints of the boundary lines per boundary region (e.g., 18 midpoints if two metal fibers are extracted, and 45 midpoints if five metal fibers are extracted). The percentage is rounded to the nearest tenth. When observing crystal grains with the

[110] orientation using TEM, electrons are transmitted through the metal fibers. However, if the fiber diameter of the metal fibers is 200 nm or more, electrons do not transmit through the metal fibers, making it impossible to obtain the desired electron diffraction pattern. Therefore, when the fiber diameter of the metal fibers is 200 nm or more, the proportion of crystal grains with the

[110] orientation is evaluated by EBSD. Although the term "boundary region when the length is divided into four equal parts along the extending direction" is used, if measurement at the boundary region is not possible for measurement reasons, measurement will be made at a portion close to the boundary region (for example, a portion within 10% of the length of the metal fiber on each side of the boundary region).

[0036] Generally, the side surfaces of metal fibers that are preferentially oriented in the

[110] direction in the longitudinal direction are easily oxidized, which is one factor that increases the resistance of the metal fibers in the width direction. In contrast, metal fibers that do not grow in the

[110] direction in the longitudinal direction are less susceptible to oxidation.

[0037] Similarly, from the viewpoint of making the metal fibers constituting the metal nonwoven fabric less susceptible to corrosion, when the length of the metal fiber is divided into four equal parts along its extension direction, the three crystals present in the boundary region preferably have a proportion of crystal grains with

[111] orientation evaluated by TEM electron diffraction or EBSD within a range of ±30° from the extension direction of the metal fiber of 50% or more, more preferably 52% or more, and even more preferably 60% or more. On the other hand, a realistic upper limit of the proportion is about 80%. The proportion of crystal grains with

[111] orientation is calculated by randomly extracting two or more metal fibers from the metal nonwoven fabric, drawing boundary lines that divide each metal fiber into four equal parts along the longitudinal direction, and measuring the three boundary regions of the boundaries. When evaluated by TEM electron diffraction, it is the percentage of crystal grains having the

[111] orientation measured at a total of six or more midpoints of the boundary lines per boundary region (for example, six midpoints when two metal fibers are extracted, and 15 midpoints when five metal fibers are extracted). When evaluated by EBSD, it is measured at a total of 18 or more midpoints of the boundary lines per boundary region (for example, 18 midpoints when two metal fibers are extracted, and 45 midpoints when five metal fibers are extracted). The percentage is rounded to the nearest tenth. Such a relationship means that the

[111] orientation is preferentially oriented in the longitudinal direction of the metal fibers. A preferred longitudinal orientation of the

[111] orientation of the crystal is preferable because, from a crystallographic perspective, it means that the (100) plane, which is susceptible to corrosion, is not exposed on the side surface. Although the term "boundary region when the length is divided into four equal parts along the extending direction" is used, if measurement at the boundary region is not possible for measurement reasons, measurement will be made at a portion close to the boundary region (for example, a portion within 10% of the length of the metal fiber on each side of the boundary region).

[0038] When the length of a metal fiber is divided into four equal parts along its extension direction, the three crystals present in the boundary regions preferably have a proportion of crystal grains with

[100] orientation,

[110] orientation, and

[111] orientation evaluated by TEM electron diffraction or EBSD within a range of ±30° along the extension direction of the metal fiber, each of which is less than 50%, more preferably 40% or less. The proportions of crystal grains with

[110] ,

[111] , and

[100] orientations are calculated by randomly extracting two or more metal fibers from a metal nonwoven fabric, drawing boundary lines that divide the length of each metal fiber into four equal parts along the longitudinal direction, and measuring the three boundary regions of the boundaries. When evaluated by TEM electron diffraction, this is the percentage of crystal grains with

[110] ,

[111] , and

[100] orientations measured at a total of six or more locations at the midpoints of the boundary lines for one boundary region (for example, six locations when two metal fibers are extracted, and 15 locations when five metal fibers are extracted). When evaluated by EBSD, measurements are made at a total of 18 or more locations, three points that divide the boundary line into four equal parts for one boundary region (for example, 18 locations when two metal fibers are extracted, and 45 locations when five metal fibers are extracted). The percentages are rounded to the nearest tenth. Note that although the term "boundary region when the length is divided into four equal parts along the extension direction" is used, if measurement at the boundary region is not possible for measurement reasons, measurements are made at a portion close to the boundary region (for example, a portion within 10% of the length of the metal fiber on each side of the boundary region).

[0039] The fact that the metal fibers contained in a metal nonwoven fabric have this relationship means that the metal crystals that make up the metal fibers are randomly oriented. The random orientation of the metal crystals that make up the metal fibers means that the metal crystals that make up the metal fibers are polycrystalline, meaning that the crystals are small. Due to the small crystal size, the temperature at which the metal fibers fuse together is lowered, allowing the metal fibers to fuse at a lower temperature, and as a result, metal nonwoven fabrics can be formed under milder conditions at lower temperatures. Furthermore, "random orientation of the crystals" means that the (100) plane is not preferentially exposed on the side surfaces of the metal fibers, meaning that oxidation of the side surfaces of the metal fibers is not promoted.

[0040] In a metal nonwoven fabric, it is preferable that the number of metal fibers having curved portions with a radius of curvature of 5 times or less the length of the metal fiber account for 20% or more of the total number of metal fibers in the metal nonwoven fabric. In this way, the metal fibers constituting the metal nonwoven fabric have inherently curved portions, which is preferable because it improves the durability of the metal nonwoven fabric when compressed, pulled, or subjected to bending stress. Furthermore, in a metal nonwoven fabric, it is easier to achieve contact across multiple metal fibers along the transverse (width) direction of the metal fibers, thereby facilitating the development of electrical and thermal conductivity. The radius of curvature is calculated as follows: First, the metal nonwoven fabric is observed using an SEM. A straight line is drawn between both ends of the metal fiber in the SEM image, and its length (chord length) is measured. Next, an auxiliary line perpendicular to the line is drawn from the midpoint of the line toward the metal fiber, and the distance (arrow height) between the midpoint and the point where it intersects with the metal fiber is measured. The radius of curvature is calculated using the following formula: r = (C × C) / (8 × h) + h / 2 (wherein r represents the radius of curvature, C represents the chord length, and h represents the arrow height.) The above-mentioned radius of curvature is preferably 0.5 μm or more and 1000 μm or less. If the metal fibers in the metal nonwoven fabric are bent, the radius of curvature is calculated from the above formula by approximating the shape of the metal fibers as having a curved portion. Furthermore, if a straight line is connected between both ends of a metal fiber and the line crosses the metal fiber, the radius of curvature is measured as if the metal fibers were different fibers, with the crossing point as the boundary.

[0041] Metal nonwoven fabrics may contain particles with shapes other than metal fibers. However, since metal nonwoven fabrics are manufactured based on the concept of controlling the gaps in the fabric by randomly depositing fibers, it is preferable that particles with shapes other than fibers are as few as possible present in the metal nonwoven fabric in order to improve the liquid permeability of the metal nonwoven fabric even when subjected to deformations such as bending and stretching. When the proportion of particles with shapes other than fibers in a metal nonwoven fabric is defined as the "irregular shape ratio," the irregular shape ratio is preferably 50% or less, more preferably 40% or less, and even more preferably 30% or less. By manufacturing metal nonwoven fabrics using the manufacturing method described below, it is possible to easily achieve an irregular shape ratio of 50% or less. The irregular shape ratio can be determined by observing the sample under SEM in a field where the diameter and length of the metal fibers are 5 to 30 times the average diameter and length of the metal fibers, and calculating the percentage of [area of ​​irregular shapes / area of ​​metal nonwoven fabric]. "Irregular shapes" refer to shapes other than fibers.

[0042] From the viewpoint of improving the liquid permeability of the metal nonwoven fabric and allowing reactants in the permeated liquid to reach the surface of the metal fibers constituting the metal nonwoven fabric in a short time, the distribution peak top diameter of voids measured by mercury porosimetry in the metal nonwoven fabric is preferably 30 μm or less, more preferably 0.01 μm or more and 30 μm or less, even more preferably 0.02 μm or more and 25 μm or less, even more preferably 0.05 μm or more and 20 μm or less, and even more preferably 0.1 μm or more and 20 μm or less. Note that when two or more peaks are observed in the void distribution measured by mercury porosimetry, it is preferable that the distribution peak top diameter of the highest peak falls within the above range, as this makes the above-mentioned advantages even more pronounced.

[0043] When the voids in the metal nonwoven fabric, i.e., the voids between the metal fibers in the metal nonwoven fabric, are 30 μm or less, the void distribution peak top diameter is determined by mercury porosimetry. When the voids exceed 30 μm, the void size is determined from the average spherical equivalent diameter of the pores divided by image analysis of X-ray CT measurement. Mercury porosimetry is measured using an Autopore IV9510 manufactured by Micromeritics. During measurement, the mercury intrusion force is set to approximately 0.5 to 60,000 psi (approximately 3 kPa to 400 MPa), the measurement mode is set to the pressure increase (intrusion) process, the mercury contact angle is set to 141.3°, and the mercury surface tension is set to 484 dyn / cm. X-ray CT measurement is performed using a SKYSCAN AN2214 manufactured by Bruker, and the average value of the sphere-equivalent diameter of the pores dividing the voids is determined using image analysis software Avizo 3D manufactured by Thermo Fisher Scientific.

[0044] The porosity of a metal nonwoven fabric can be calculated as the volume of the voids present within the metal nonwoven fabric relative to the volume based on the dimensions of the outer contour of the metal nonwoven fabric. Specifically, first, the volume V1 of the metal nonwoven fabric is calculated from the length and width of the metal nonwoven fabric and the thickness of the metal nonwoven fabric measured using the method described below. Next, the mass of the metal nonwoven fabric is measured, and the volume V2 of the metal nonwoven fabric is calculated from the specific gravity of the metals constituting the metal nonwoven fabric. The volume V3 of the voids present within the metal nonwoven fabric is calculated as a percentage of the volume V1 of the metal nonwoven fabric, and this is the void ratio. From the viewpoint of increasing the amount of liquid that permeates the metal nonwoven fabric, the void ratio is preferably 50% or more, more preferably 60% or more, even more preferably 70% or more, and even more preferably 80% or more. Furthermore, from the viewpoint of maintaining the strength of the metal nonwoven fabric, the void ratio is preferably 99% or less.

[0045] The distribution peak top diameter of the pores in the above-mentioned metal nonwoven fabric measured by mercury porosimetry, the average equivalent sphere diameter of the pores dividing the pores determined by X-ray CT, and the porosity can be achieved, for example, by adjusting the average fiber diameter or average length of the metal fibers. They can also be achieved by intentionally mixing multiple types of metal fibers with different average fiber diameters or average lengths. Additionally or alternatively, they can also be achieved by appropriately adjusting the conditions in the manufacturing method of the metal nonwoven fabric described below.

[0046] A metal nonwoven fabric having such a structure has appropriate voids within the fabric, which, due to factors such as the size of the voids, can improve the liquid permeability of the fabric, particularly that of aqueous liquids, by capillary action resulting from the size of the voids. In other words, the amount of liquid absorbed per unit volume of the dry metal nonwoven fabric can be increased. For example, when a solution containing a reactant comes into contact with the fabric, the solution quickly penetrates the voids in the fabric, allowing the reactant to easily reach the surface of the metal fibers forming the voids or their immediate vicinity. Furthermore, because the voids within the fabric are sufficiently narrow relative to the diffusion rate of the reactant in the solution, the reactant can quickly reach the surface of the metal fibers that make up the fabric. On the other hand, when a solution containing a reactant comes into contact with a metal nonwoven fabric having voids wider than the appropriate voids, capillary action does not occur as effectively as with the metal nonwoven fabric of the present invention, and the solution does not fully penetrate the fabric. In other words, the liquid permeability of the liquid cannot be improved. Furthermore, because the voids within the metal nonwoven fabric are large relative to the diffusion rate of the reactant in the solution, the reactant is unlikely to reach the surface of the metal fibers that make up the metal nonwoven fabric in a short time. Therefore, when the metal nonwoven fabric of the present invention is brought into contact with a solution, the reactant contained in the solution is more likely to react efficiently, making it easier to exhibit desired physical and chemical properties. In this specification, "aqueous liquid" refers to a liquid (such as a solution, dispersion, or emulsion) containing 5% or more by mass of water. The advantages of the metal nonwoven fabric of the present invention are not limited to water-based liquids, but are also exhibited with organic solvent-based liquids, regardless of the type of solvent. This is because treating the surfaces of the metal fibers that make up the metal nonwoven fabric with a substance that has affinity for the solvent that will come into contact with the metal nonwoven fabric can impart liquid permeability to various solvents.

[0047] From the viewpoint of improving the liquid permeability of the metal nonwoven fabric, improving the reactivity with the liquid when the metal nonwoven fabric is used as, for example, an electrode, and satisfying practicality as an electrode, the metal nonwoven fabric preferably has an average thickness of 3 μm or more, more preferably 50 μm or more, and even more preferably 90 μm or more. By having an average thickness of 3 μm or more, the metal nonwoven fabric can maintain a self-supporting state. Furthermore, the upper limit of the average thickness of the metal nonwoven fabric cannot be determined in general because it depends on the application, but it may be 50 mm or less, 30 mm or less, 10 mm or less, 5 mm or less, or 2 mm or less.

[0048] The average thickness of a metal nonwoven fabric can be measured by the following method. First, a piece of the metal nonwoven fabric is cut into a length of 2 cm and a width of 2 cm to prepare a cut piece of the metal nonwoven fabric. However, if it is not possible to prepare a cut piece of this size as the cut piece to be measured, a cut piece as large as possible is prepared. Next, the cut piece is placed on a flat plate, and a flat plate weighing 1 g and having dimensions larger than the cut piece is placed on top of it. In this state, the distance from the lower surface of the flat plate below the cut piece to the upper surface of the flat plate above the cut piece is measured with a vernier caliper. The thicknesses of the two flat plates measured in advance are subtracted from the obtained measurement value, and this is the thickness of the metal nonwoven fabric. The thicknesses of cut pieces taken from any three or more locations of the metal nonwoven fabric are measured, and the arithmetic mean value of these is taken as the average thickness. Note that if the thickness of the metal nonwoven fabric exceeds 10 μm, it is measured with a vernier caliper as described above. If it is 10 μm or less, the average thickness is measured using a cross-sectional SEM image.

[0049] When the liquid is allowed to penetrate the metal nonwoven fabric and the reactant in the liquid reaches the surface of the metal fibers constituting the metal nonwoven fabric in a short time, the reaction on the surface of the metal fibers is caused to occur just enough. Therefore, the metal nonwoven fabric has a specific surface area of ​​0.02 m ２ / g or more 22m ２ / g or less, and ２ / g or more 10m ２ / g or less, and more preferably 0.2m ２ / g or more 3m２ / g or less, and more preferably 0.25m ２ / g or more 2m ２ It is even more preferable that the SiO2 content is 1 / g or less.

[0050] The specific surface area of ​​a metal nonwoven fabric can be measured by the following method. ２ For metal nonwoven fabrics with a specific surface area of ​​0.1 m / g or more, the specific surface area is measured by the krypton gas adsorption-BET multipoint method using, for example, a specific surface area / pore distribution measuring device BELSORP-max manufactured by BEL Japan. ２ For metal nonwoven fabrics with a specific surface area of ​​less than 1 / g, X-ray CT measurement is performed using, for example, a nanofocus X-ray CT scanner SKYSCAN AN2214 manufactured by Bruker, and the specific surface area is determined using image analysis software Avizo 3D manufactured by Thermo Fisher Scientific, which quantifies the structural characteristics of the three-dimensional object being measured, such as the area and volume of the object, based on the obtained three-dimensional information.

[0051] The specific surface area of ​​the metal nonwoven fabric can be adjusted, for example, by forming the metal nonwoven fabric using metal fibers with different average fiber diameters, or by laminating multiple types of metal nonwoven fabric.

[0052] In order to allow the liquid to penetrate the metal nonwoven fabric and for the reactant in the liquid to reach the surface of the metal fiber in a short time, and to allow the reaction to occur on the surface of the metal fiber in an adequate amount, the metal nonwoven fabric has an apparent density of 0.01 g / cm ３ 2.0g / cm or more ３ It is preferable that the density is 0.05 g / cm or less. ３ 1.7g / cm or more ３ More preferably, it is 0.1 g / cm or less. ３ 1.4g / cm or more ３ The apparent density is determined by calculating the volume V1 of the metal nonwoven fabric and the mass of the metal nonwoven fabric using the above-mentioned method, and then dividing the mass by the volume V1.

[0053] The apparent density of the metal nonwoven fabric can be adjusted, for example, by increasing the average fiber diameter of the metal fibers, increasing the average thickness of the metal fibers, or by laminating multiple types of metal nonwoven fabric.

[0054] Next, a preferred method for producing the nonwoven metal fabric of the present invention will be described. An electrolytic method is preferably used to produce the metal fibers that primarily constitute the nonwoven metal fabric. By selecting appropriate additives and using electrolytic methods under appropriate electrolytic conditions, metal fibers with a low degree of agglomeration can be easily obtained. When such metal fibers are deposited and sintered, they tend to sinter at low temperatures. As a result, nonwoven metal fabric can be easily formed. In addition, the electrolytic method has the advantage of easily controlling the metal fibers into the desired shape. Furthermore, since the electrolyte can be reused, less liquid is required to produce the nonwoven metal fabric, and at the same time, the amount of waste liquid to be treated can be reduced.

[0055] When producing metal fibers by electrolysis, for example, an anode and a cathode are immersed in a sulfuric acid electrolyte containing a metal element source, and a direct current is passed through the electrolyte to perform electrolytic reduction, thereby depositing metal fibers on the surface of the cathode. ２ More than 600A / m ２ Thereafter, when producing a metal nonwoven fabric, the slurry of the electrolytic solution containing the precipitated metal fibers is poured into a mold and allowed to settle, the supernatant liquid is removed, the mixture is dried, and if necessary, pressure is applied to carry out reduction.

[0056] There are no particular limitations on the type of metal element used in the present production method, as long as it is possible to produce metal fibers by the present production method, and examples thereof include the metal elements described above.

[0057] When producing metal fibers using the above procedure, it is preferable to carry out electrolytic reduction with an oily substance attached to the surface of the cathode. By reducing the metal ions in this state, it is possible to control the electrolytic reduction reaction that occurs on the electrode.

[0058] Oily substances to be applied to the cathode surface during metal fiber production include various organic compounds that are poorly soluble or insoluble in water and have a viscosity sufficient to allow the substance to be retained on the cathode surface after application. Such organic compounds, provided that they are poorly soluble or insoluble in water, include aliphatic hydrocarbons, aromatic hydrocarbons, aliphatic alcohols, aromatic alcohols, aliphatic aldehydes, aromatic aldehydes, aliphatic ethers, aromatic ethers, aliphatic ketones, aromatic ketones, aliphatic carboxylic acids and their salts, aromatic carboxylic acids and their salts, amides of aliphatic carboxylic acids, amides of aromatic carboxylic acids, esters of aliphatic carboxylic acids, and esters of aromatic carboxylic acids. Among these, higher fatty acids or their salts, esters, or amides, or organic solvents containing a mixture thereof, are particularly preferred. The higher fatty acids may be monobasic or polybasic. Examples of higher fatty acids include saturated or unsaturated aliphatic carboxylic acids, preferably having 10 to 25 carbon atoms, more preferably 10 to 22 carbon atoms, and even more preferably 11 to 20 carbon atoms.

[0059] Methods for adhering an oily substance to the surface of the cathode described above include, for example, a method of directly applying the oily substance to the surface of the cathode, a method of adhering the oily substance by immersing the cathode in a container containing the oily substance, and a method of floating the oily substance on the surface of the electrolyte and immersing the cathode from above to adhering the oily substance to the cathode surface. Another method involves suspending the oily substance in an electrolyte containing a cathode, stirring the suspended electrolyte, so that the suspended oily substance collides with the surface of the cathode and adheres directly to the surface of the cathode. Furthermore, if the oily substance has the property of being dissolved in a small amount in the electrolyte, even if the suspended oily substance does not directly contact the electrode, the oily substance once dissolved in the electrolyte will be continuously adsorbed to the electrode surface, resulting in an effect similar to that achieved when adhering to the surface of the cathode.

[0060] The amount of oily substance to be attached to the surface of the cathode when manufacturing the metal fiber is 1 g / m per unit surface area of ​​the cathode. ２ More than 500g / m ２It is preferable that the density is 3 g / m or less. ２ More than 200g / m ２ It is more preferable that the concentration is 5 g / m or less. ２ More than 100g / m ２ It is more preferable that:

[0061] Next, the resulting slurry containing metal fibers is allowed to stand, allowing the metal fibers to settle, producing a metal nonwoven fabric. Specifically, the slurry containing the precipitated metal fibers is poured into a water-impermeable mold, and the mold is allowed to stand, allowing the metal fibers to settle. The mold dimensions may be approximately the same as or larger than the dimensions of the desired metal nonwoven fabric. By adjusting the amount of slurry poured into the mold and the concentration of metal fibers contained in the slurry, a metal nonwoven fabric with the desired liquid permeability, pore size, and porosity can be obtained. Alternatively, the metal nonwoven fabric can be produced by filtering the slurry containing metal fibers through a filter and drying it as is, followed by removing the filter after drying. Alternatively, a metal nonwoven fabric can be produced by spraying the slurry containing dispersed metal fibers onto a metal substrate or other substrate to form a film, or by using a spin coater. In the above-mentioned film-forming method, the structure of the metal nonwoven fabric can be controlled by applying an electric or magnetic field simultaneously with film formation. The metal fibers may have their surfaces coated with a material other than metal. Alternatively, the metal fibers may have a core made of metal and a shell made of a metal other than the core and disposed on the surface of the core. After producing a metal nonwoven fabric, the metal nonwoven fabric may be coated with a material other than metal. Alternatively, after producing a metal nonwoven fabric, the metal nonwoven fabric may be coated with a metal different from the metal constituting the metal nonwoven fabric.

[0062] After the metal fibers are deposited in the mold, the supernatant liquid of the slurry is removed from the mold by, for example, using a centrifuge, decanting the supernatant liquid, or volatilizing the supernatant liquid.

[0063] After removing the supernatant liquid from the mold, the deposit remaining in the mold is dried under atmospheric pressure or in an inert gas atmosphere. During this process, the deposit is not completely dried, and a small amount of supernatant liquid still remains. Furthermore, since the metal fibers in the deposit are very thin and long, voids are formed in the deposit.

[0064] Thereafter, if necessary, the deposit can be pressed in the thickness direction using a press. This process allows the desired thickness of the metal nonwoven fabric to be adjusted. Furthermore, by adjusting the pressure, time, temperature, etc., when pressing the deposit in the thickness direction, a metal nonwoven fabric having the desired liquid permeability, void size, and porosity can be obtained. When pressing the deposit in the thickness direction, for example, the deposit can be removed from the mold, surrounded on all four sides by a metal plate (spacer) having the desired thickness, and pressure can be applied from above the metal plate. By adjusting the amount of slurry poured into the mold and the concentration of metal fibers contained in the slurry, a metal nonwoven fabric having the desired liquid permeability, void size, and porosity can be obtained. The pressure when pressing the deposit is preferably 0.01 MPa or more and 50 MPa or less, more preferably 1 MPa or more and 30 MPa or less, and even more preferably 1 MPa or more and 20 MPa or less, from the viewpoint of obtaining a metal nonwoven fabric having the desired liquid permeability, void size, and porosity. From the same viewpoint as above, the time for pressing the deposit is preferably 3 minutes to 5 hours, more preferably 10 minutes to 3 hours, and even more preferably 30 minutes to 2 hours. When pressing the deposit, the deposit may be heated. Alternatively, the deposit may be pressed without heating. When the deposit is heated, the temperature is preferably 500°C or less, more preferably 350°C or less, from the viewpoint of preventing excessive fusion between the metal fibers.

[0065] After drying the deposit or pressing the deposit in the thickness direction, the deposit is calcined without pressure in an argon, nitrogen, or hydrogen-containing atmosphere, as needed, to fuse the intersections of the metal fibers in the deposit and obtain a metal nonwoven fabric. If it is necessary to reduce the surfaces of the metal fibers during the fusion process, the deposit is calcined without pressure in a hydrogen-containing atmosphere. Because the metal fibers are very long and thin, the surfaces of the metal fibers in the deposit are easily oxidized. Therefore, it is advantageous to calcinate the deposit to reduce the surfaces of the metal fibers. Furthermore, when metal fibers are obtained by the above-mentioned electrolytic method, calcining the deposit for reduction has the advantage of facilitating at least a portion of the metal fibers in the deposit to fuse. The calcination time for the deposit is preferably 3 minutes to 24 hours, more preferably 30 minutes to 5 hours, from the viewpoint of sufficiently and efficiently reducing the surface oxides and fusing the metal fibers. The temperature at which the deposit is calcined is preferably 800°C or lower, and more preferably 150°C or higher and 500°C or lower, from the viewpoint of efficiently reducing the metal and achieving adequate fusion between the metal fibers. The metal nonwoven fabric obtained in this manner may be cut into a desired shape. Furthermore, a plurality of the obtained metal nonwoven fabrics may be stacked to form a metal nonwoven fabric having a laminated structure. In addition to the above-mentioned heat treatment, after forming the metal nonwoven fabric, the entire fabric may be metal-plated using the metal nonwoven fabric as an electrode. This allows the surface of the metal nonwoven fabric to be coated with a different type of metal, or the same type of metal to be plated, thereby improving the mechanical strength of the metal nonwoven fabric and controlling the size and porosity of the voids within the metal nonwoven fabric.

[0066] By producing a metal nonwoven fabric using the above method, the metal nonwoven fabric as a whole becomes liquid permeable, allowing the reactant in the liquid to reach the surface of the metal fibers that make up the metal nonwoven fabric in a short period of time, resulting in the production of a metal nonwoven fabric with a reaction area where a reaction occurs immediately.

[0067] In the metal fibers in the metal nonwoven fabric, a polycrystalline structure is formed in which multiple crystals are connected along the longitudinal direction. Furthermore, the

[110] orientation of the crystals is less likely to be preferentially oriented in the longitudinal direction. Furthermore, the

[111] orientation of the crystals is more likely to be preferentially oriented in the longitudinal direction, or the orientation direction of the crystals is more likely to be random. As described above, when a metal other than the metal is laminated on the surface of the core portion made of metal in the metal fiber, the metal other than the metal may or may not be oriented in this way.

[0068] The metal nonwoven fabric of the present invention obtained by the above method can be used as an electrochemical material, taking advantage of the liquid permeability of the metal nonwoven fabric and the physical and chemical properties of the metal fibers. For example, the metal nonwoven fabric of the present invention can be cut to a desired size and used as an electrode. Alternatively, the metal nonwoven fabric of the present invention can be combined with fibrous or granular materials made of the same or different materials as the nonwoven fabric to form an electrode. Electrodes formed from metal nonwoven fabrics have more voids than, for example, electrodes formed from metal plates, thereby improving reactivity with liquids. As a result, when the metal nonwoven fabric of the present invention is used as an electrode, it has improved liquid permeability compared to conventional porous materials that have been widely used as electrode materials for liquid reactions. This allows reactants to reach the surface of the metal fibers that make up the metal nonwoven fabric in a shorter time, resulting in even better reactivity. Therefore, the metal nonwoven fabric of the present invention can be used, for example, as an electrode for a biosensor, an electrode for water electrolysis, an electrode for carbon dioxide decomposition, an electrode for organic synthesis, an electrode for hazardous substance decomposition, an electrode for valuable substance synthesis, an electrode for valuable substance extraction, a current collector for various batteries, and the like. Among these, electrodes made of metal nonwoven fabric are a preferred example for use as electrodes for biosensors, as they can react quickly with a small amount of liquid.

[0069] Furthermore, when the metal nonwoven fabric of the present invention obtained by the above method is used as a selective adsorbent for chemical substances, the metal nonwoven fabric also functions advantageously in reaction systems in which a reactant in a liquid reacts with metal fibers in the metal nonwoven fabric or with components applied to the surface of the metal fibers without the application of an external current. Specifically, when an aqueous solution containing dissolved hydrogen sulfide is contacted with the surface of a metal such as copper, silver, gold, nickel, lead, palladium, platinum, cobalt, tin, iron, zinc, or bismuth, sulfide ions in the aqueous solution react with the surface of the metal to produce sulfide, thereby removing harmful sulfide ions from the aqueous solution. When the metal nonwoven fabric of the present invention is used as the metal to be reacted with the aqueous solution, the metal nonwoven fabric improves liquid permeability and allows reactants in the permeated liquid to easily reach the surface of the metal fibers that make up the metal nonwoven fabric, enabling more efficient removal of sulfide ions. The metal nonwoven fabric of the present invention can also be used to efficiently remove mercury from an aqueous solution, for example, using a similar method. The same effect can be achieved in articles that contain the nonwoven metal fabric of the present invention in part. Therefore, the nonwoven metal fabric of the present invention can be used, for example, in filters for removing harmful substances from wastewater.

[0070] It is also known that the antibacterial properties of metals such as copper can inhibit the growth of bacteria and viruses in liquids and gases. Therefore, by incorporating an antibacterial metal as part of the constituent materials of the metal nonwoven fabric of the present invention, the frequency of contact between the target bacteria or viruses and the metal fiber surface can be increased, thereby improving the efficiency of the antibacterial effect. Therefore, the metal nonwoven fabric of the present invention can be used, for example, in filters that sterilize bacteria and viruses in wastewater from medical institutions without heating, or in air purifier filters that remove airborne infectious viruses. Furthermore, by a similar mechanism, the heat exchange efficiency can be improved when the metal nonwoven fabric of the present invention is used as the heat transfer portion of a heat exchanger that uses a liquid as a medium. Therefore, the metal nonwoven fabric of the present invention can be used, for example, in effective water-cooled chillers for electronic devices.

[0071] Although the present invention has been described above based on the preferred embodiments, the present invention is not limited to the above embodiments.

[0072] The above-described embodiments of the present invention encompass the following technical concepts: [1] A liquid-permeable metal nonwoven fabric comprising metal fibers, wherein the metal fibers have an average fiber diameter of 20 nm to 10 μm, and the void distribution peak top diameter measured by mercury porosimetry is 30 μm or less. [2] The metal nonwoven fabric according to [1], wherein the void distribution peak top diameter measured by mercury porosimetry is 0.01 μm or more. [3] The metal nonwoven fabric according to [1] or [2], wherein, for metal crystals constituting the metal fibers, when the length along the extension direction of the metal fibers is X and the length along the direction perpendicular to said direction is Y, the arithmetic mean value of X / Y, which is the ratio of X to Y at three boundary regions when the length along the extension direction of the metal fibers is divided into four equal parts, is 4 or less. [4] The metal nonwoven fabric according to any one of [1] to [3], wherein the average length of the metal fibers is 0.5 μm or more and 5000 μm or less. [5] The metal nonwoven fabric according to any one of [1] to [4], wherein the average thickness of the metal nonwoven fabric is 3 μm or more. [6] The metal nonwoven fabric according to any one of [1] to [4], wherein the specific surface area is 0.02 m ２ / g or more 22m ２ / g or less. [7] The metal nonwoven fabric according to any one of [1] to [5], wherein the metal constituting the metal fibers is copper, silver, gold, nickel, lead, palladium, platinum, cobalt, tin, iron, zinc, or bismuth, or an alloy containing these metals. [8] The metal nonwoven fabric according to any one of [1] to [7], wherein the surfaces of the metal fibers are coated with a material other than metal. [9] The metal nonwoven fabric according to any one of [1] to [8], wherein the metal fibers comprise a core portion made of metal and a shell portion disposed on the surface of the core portion and made of a metal other than the metal.

[10] An electrode made using the metal nonwoven fabric according to any one of [1] to [9].

[0073] The present invention will be described in more detail below with reference to examples. However, the scope of the present invention is not limited to such examples. Unless otherwise specified, "%" means "% by mass."

[0074] Example 1 In this example, a metal nonwoven fabric made of copper was produced. An electrolyte solution was prepared from copper sulfate and sulfuric acid so that the copper ion concentration was 40 g / L and the free sulfuric acid concentration was 19.6 g / L. 800 mL of this solution was placed in an electrolytic cell measuring 10 cm wide x 8 cm deep x 12 cm deep (capacity: approximately 1000 mL) and stirred. The temperature of the electrolyte solution was set to 40°C. An 8 cm x 8 cm copper plate was used as the cathode. Oleic acid was uniformly applied to the surface of the cathode. The amount of oleic acid applied was 7 g / m. ２ An 8 cm x 8 cm copper plate was used as the anode. The cathode and anode were suspended in the electrolytic cell so that the gap between them was 8 cm. The current density was 313 A / m ２ The electrolysis was carried out for 30 minutes with the temperature adjusted to 50°C. In this way, fibrous copper was electrodeposited on the surface of the cathode. The deposited fibrous copper was deposited on the electrode with a low degree of aggregation, so it was peeled off and dispersed in 2-propanol to form a slurry. 2.34 mL of the slurry (fibrous copper concentration 100 g / L) was poured into a mold (length 50 mm × width 12 mm × height 15 mm) with a Ti plate as the bottom plate, and the mold was left to stand at room temperature (25°C) for 1 hour to deposit the precipitate. The supernatant was removed, and the remaining organic solvent was volatilized to remove the solvent from the mold. The deposit remaining in the mold was dried in a vacuum atmosphere at room temperature (25°C) for 12 hours. Thereafter, the mixture was immersed in a hydrogen-containing atmosphere (3 vol% H ２ Remainder N ２ The deposit was calcined under a pressureless atmosphere to obtain a metal nonwoven fabric. The calcination was carried out at 350°C for 1.5 hours. When the obtained metal nonwoven fabric was observed using an SEM, metal fibers constituting the metal nonwoven fabric were observed. Each end of the metal fibers was tapered and curved.

[0075] Comparative Example 1 A Cu (copper) metal porous body (MF-30) manufactured by Nagamine Manufacturing Co., Ltd. was used as the metal nonwoven fabric in Comparative Example 1. When MF-30 was observed using an SEM, it was found to be a sponge-like porous body.

[0076] Comparative Example 2 A Cu (copper) porous metal material (MF-80A) manufactured by Nagamine Manufacturing Co., Ltd. was used as the metal nonwoven fabric in Comparative Example 2. When MF-80A was observed using an SEM, it was found to be a sponge-like porous material.

[0077] [Example 2] 0.78 mL of the same slurry prepared in Example 1 was placed in a mold with a Ti plate as the bottom plate, in the same manner as in Example 1. The mold containing the deposit was dried under vacuum in the same manner as in Example 1, and the deposit was then removed from the mold. The deposit was surrounded on all four sides by 100 μm thick copper plates, and the deposit was pressed in the thickness direction from above the plates at 20 MPa. The deposit was pressed at room temperature (25°C) for 0.5 hours. A metal nonwoven fabric was obtained in the same manner as in Example 1, except for this.

[0078] [Example 3] An electrolyte solution was prepared from copper sulfate and sulfuric acid so that the copper ion concentration was 10 g / L and the free sulfuric acid concentration was 5 g / L. The current density was 156 A / m ２ Electricity was passed through the electrode for 30 minutes. In this way, fibrous copper was electrodeposited on the surface of the cathode. The fibrous copper was dispersed in 2-propanol to form a slurry. 4.54 mL of this slurry (concentration: 27 g / L) was filtered through a 45 mm silica filter (inner diameter: 35 mm). The filter was removed while still attached to the filter, and the remaining solvent was evaporated. Thereafter, drying under a vacuum atmosphere and firing under a hydrogen-containing atmosphere were carried out in the same manner as in Example 1. Finally, the silica filter was peeled off from the molded copper nonwoven fabric.

[0079] [Comparative Example 3] A copper fiber sheet manufactured by Tomoegawa Paper Co., Ltd. was used as the metal nonwoven fabric in Comparative Example 3. When the copper fiber sheet was observed using an SEM, it was found to be a porous body composed of copper fibers. Since adhesive tape was attached, the copper fiber sheet was immersed in ethanol to separate the adhesive tape.

[0080] Example 4: An electrolyte solution was prepared from copper sulfate and sulfuric acid to a copper ion concentration of 40 g / L and a free sulfuric acid concentration of 19.6 g / L. 800 mL of this solution was placed in an electrolytic cell measuring 10 cm wide x 8 cm deep x 12 cm deep (capacity approximately 1000 mL) and stirred. Instead of applying oleic acid to the surface of the cathode, 30 mL of oleic acid was added to the electrolyte solution. The electrolyte solution was stirred to suspend the oleic acid throughout the electrolyte solution. The temperature of the electrolyte solution was set to 40°C. A metal nonwoven fabric was otherwise obtained in the same manner as in Example 1. The obtained metal nonwoven fabric was observed using an SEM, revealing metal fibers constituting the metal nonwoven fabric. Each end of the metal fiber was tapered and curved.

[0081] [Evaluation 1] The average fiber diameter and average length of the metal fibers constituting the samples obtained in Examples 1 to 4 and the sample prepared as Comparative Example 3 were measured using the method described above. Note that the samples used in Comparative Examples 1 and 2 were sponge-like porous bodies (hereinafter also referred to as "sponge metals"), so measurements were omitted. Furthermore, the X / Y values ​​for the metal crystals constituting the metal fibers in the samples obtained in Examples 1 to 4 were calculated using the method described above. Note that the X / Y values ​​were not calculated in Comparative Examples 1 to 3. This is because the samples used in Comparative Examples 1 and 2 were sponge-like metals, and the X / Y values ​​do not reflect the crystal shape. The sample used in Comparative Example 3 had a large average fiber diameter of 20 μm, so, like Comparative Examples 1 and 2, the X / Y values ​​do not reflect the crystal shape. In addition, for the samples obtained in Examples 1 to 4 and Comparative Examples 1 to 3, the pore distribution peak top diameter was measured by the mercury porosimetry method using the above-mentioned method (when the pore size was 30 μm or less), or the average sphere-equivalent diameter of the pores divided by image analysis of X-ray CT measurement was measured using the above-mentioned method (when the pore size was more than 30 μm), to determine the size of the pores in the metal nonwoven fabric constituting the sample or the size of the pores in the sponge-like metal. Also, the porosity was measured using the above-mentioned method. Furthermore, for the samples obtained in Examples 1 to 4 and Comparative Examples 1 to 3, the average thickness, apparent density, and specific surface area of ​​the sample were measured using the above-mentioned method. When the specific surface area was 0.1 m ２For metal nonwoven fabrics with a specific surface area of ​​0.1 m / g or more, the specific surface area was measured by the krypton gas adsorption-BET multipoint method using a BELSORP-max specific surface area / pore distribution measuring device manufactured by BEL Japan, as described above. ２ For metal nonwoven fabrics with a specific surface area of ​​less than 1 / g, the specific surface area was measured by X-ray CT using a Bruker nanofocus X-ray CT scanner SKYSCAN AN2214. The apparent density of the sample was determined using the above-mentioned method from the volume V1 and mass of the sample. These results are shown in Table 1 below. Note that the length of the metal fibers in the sample of Comparative Example 3 was too long to be measured using the above-mentioned method, so it is listed as "continuous" in Table 1.

[0082] [Evaluation 2] The liquid permeability of the samples of Examples 1 to 4 and Comparative Examples 1 to 3 was evaluated according to the method described above. The average amount of water absorbed per volume based on the outer contour dimensions of the sample in a dry state was calculated, and if this value was 0.4 mL / cm ３ A value of 0.4 mL / cm or more was evaluated as having liquid permeability. ３ Those for which the permeability was less than 1 / 2 were evaluated as having no liquid permeability. The results are shown in Table 1. The commercially available samples of Comparative Examples 1 to 3 were immersed in a 0.1 mol / L aqueous sodium hydroxide solution for 5 minutes before measurement, and then immersed in a 4-fold diluted aqueous nitric acid solution for 3 minutes to clean the surface.

[0083] [Evaluation 3] Evaluation was carried out to confirm the reach of reactants to the metal fibers or surfaces constituting the samples of Examples 1 to 4 and Comparative Examples 1 to 3. Using the samples of Examples 1 to 4 and Comparative Examples 1 to 3, the quantity of electricity (electrochemically detected quantity of electricity) (mC) required for oxidation in the electrolysis of glucose in an aqueous solution was determined by the method described below. The results are shown in Table 1.

[0084] [Measurement of the Quantity of Electricity Required for Glucose Oxidation] The samples of Examples 1 to 4 and Comparative Examples 1 to 3 were cut into pieces measuring 5 x 20 mm and placed on a 48 x 30 mm plastic plate with a glass fiber filter (GA-100, manufactured by ADVANTEC Corporation) on top. The glass fiber filter was pre-cut to the size of the plastic plate. The commercially available samples of Comparative Examples 1 to 3 were immersed in a 1 mol / L aqueous sodium hydroxide solution for 5 minutes before measurement, and then immersed in a 4x diluted nitric acid solution for 3 minutes to clean the surface. The samples of Examples 1 to 4 were also cleaned before measurement, but the immersion in the 4x diluted nitric acid solution was for 15 seconds. The sample of Example 3 was also cleaned before measurement, but was not immersed in the 4x diluted nitric acid solution. These samples were used as detection electrodes for detecting glucose. The average thickness of each cut sample (metal nonwoven fabric or sponge metal) was as shown in Table 1. A platinum wire with a diameter of approximately 0.5 mm was placed along the longitudinal direction of the detection electrode, approximately 4.5 mm away from the side edge of the detection electrode, and served as the counter electrode. A 48 mm x 25 mm, 1.5 mm thick silicone rubber plate with an oval hole measuring 15 mm wide and 10 mm long was placed on top of the detection electrode, and a 0.8 mm thick plastic plate with a hole of the same size as the silicone rubber was placed on top. The sample was positioned so that the side edge (the side with the counter electrode) overlapped the line indicating the minor axis of the oval hole and secured with a clip. A silver / silver chloride electrode with an outer diameter of 3 mm and a ceramic plug embedded at the tip was used as the standard electrode, and was placed on the detection electrode side between the detection electrode and the counter electrode. The tip of the standard electrode was lightly touched to the surface of the glass fiber filter. Electrical connection to the detection electrode was made using an alligator clip to connect the portion of the standard electrode protruding from the plastic plate that served as the cover. Before detection, 1000 μL of 0.1 mol / L sodium hydroxide aqueous solution was added to the detection electrode, standard electrode, and silver / silver chloride electrode to wet the three electrodes and establish electrical continuity between them. Next, a voltage of −0.2 to 0.8 V (vs. the silver / silver chloride standard electrode) was applied to the detection electrode at 50 mV / s, and this operation was repeated five times to form an oxide film on the copper surface of the detection electrode.This procedure was repeated five times. If the difference in current between the last and previous 0.8 V potential repetitions was greater than 0.05 mA, this procedure was repeated again. This procedure was repeated until the difference in current between the last and previous 0.8 V potential repetitions was less than 0.05 mA. A twisted paper towel made with a JK wiper (manufactured by Nippon Paper Crecia Co., Ltd.) was then placed on the top surface of the detection electrode to absorb any solution seeping out of the electrode. The underlying glass fiber filter remained wet. Next, electricity was applied to the detection electrode until the potential reached 0.55 V (vs. a silver / silver chloride standard electrode). 20 μL of 0.1 mol / L aqueous sodium hydroxide solution was added as an auxiliary electrolyte. A predetermined amount of glucose (5440 μmol / L) dissolved in 0.1 mol / L aqueous sodium hydroxide solution was then dropped onto the detection electrode using a micropipette. When the detection electrode was approximately 1 mm thick, as in Examples 1 and 4 and Comparative Examples 1 and 2, 20 μL of the sodium hydroxide aqueous solution was added to the detection electrode. When the detection electrode was approximately 0.1 mm thick, as in Examples 2 and 3 and Comparative Example 3, 2 μL of the sodium hydroxide aqueous solution was added to the detection electrode. As glucose permeated the detection electrode, a catalytic reaction occurred between the glucose and the copper oxide present on the surface of the detection electrode, oxidizing the glucose and causing an oxidation current to flow between the detection electrode and the counter electrode. Therefore, the amount of electricity required for glucose oxidation could be detected. The time-dependent change in the oxidation current, which began to flow the moment the sodium hydroxide aqueous solution containing dissolved glucose was dripped, was monitored. The amount of electricity detected within 30 seconds after the sodium hydroxide aqueous solution containing dissolved glucose was dripped, was determined. In Examples 1 and 4 and Comparative Examples 1 and 2, when 20 μL of the sodium hydroxide aqueous solution was added to the detection electrode having a thickness of about 1 mm, the peak of the oxidation current continued even 30 seconds after the dropping, but the integration of the electrical quantity was uniformly terminated after 30 seconds.

[0085]

[0086] As is clear from Table 1, when comparing Examples 1 and 4 with Comparative Examples 1 and 2, which have approximately the same average thickness, the metal nonwoven fabrics obtained in Examples 1 and 4 have higher liquid permeability than the samples obtained in Comparative Examples 1 and 2, and the electrical quantity values ​​for electrochemically detecting glucose are significantly improved. Furthermore, when comparing Examples 2 and 3 with Comparative Example 3, which have similar average thickness, the metal nonwoven fabrics obtained in Examples 2 and 3 have higher liquid permeability than the sample obtained in Comparative Example 3, and the electrical quantity values ​​for electrochemically detecting glucose are significantly improved. This demonstrates that the metal nonwoven fabrics obtained in Examples 1 to 4 have higher liquid permeability than the samples in Comparative Examples 1 to 3. Furthermore, unlike the samples in Comparative Examples 1 to 3, the metal nonwoven fabrics obtained in Examples 1 to 4 are composed of thin and long metal fibers. This demonstrates that the reactant in the liquid reaches the surface of the metal fibers that make up the metal nonwoven fabric in a short time, allowing the desired reaction to proceed effectively. Therefore, it can be seen that the metal nonwoven fabrics obtained in Examples 1 to 4 have better electrode performance than the samples in Comparative Examples 1 to 3.

[0087] According to the present invention, it is possible to provide a metal nonwoven fabric that has improved liquid permeability and that has the function of allowing reactants in the permeated liquid to easily reach the surface of the metal fibers that make up the metal nonwoven fabric.

Claims

1. A liquid-permeable metal nonwoven fabric comprising metal fibers, wherein the average fiber diameter of the metal fibers is 20 nm or more and 10 μm or less, and the void distribution peak top diameter measured by mercury porosimetry is 30 μm or less.

2. The nonwoven metal fabric according to claim 1, wherein the void distribution peak top diameter measured by mercury porosimetry is 0.01 μm or more.

3. The metal nonwoven fabric according to claim 1, wherein the length of the metal crystals constituting the metal fibers is defined as X in the direction of extension of the metal fibers and as Y in the direction perpendicular to said direction, and the arithmetic mean value of X / Y, which is the ratio of X to Y at three boundary regions when the length of the metal fibers is divided into four equal parts along the direction of extension of the metal fibers, is 4 or less.

4. The metal nonwoven fabric according to claim 1, wherein the average length of the metal fibers is 0.5 μm or more and 5000 μm or less.

5. The metal nonwoven fabric according to claim 1, wherein the average thickness of the metal nonwoven fabric is 3 μm or more.

6. Specific surface area is 0.02 m ２ / g or more 22m ２ 2. The nonwoven metal fabric according to claim 1, wherein the elastic modulus is 1 / g or less.

7. The metal nonwoven fabric according to claim 1, wherein the metal constituting the metal fibers is copper, silver, gold, nickel, lead, palladium, platinum, cobalt, tin, iron, zinc, or bismuth, or an alloy containing any of these metals.

8. The metal nonwoven fabric according to claim 1, wherein the surface of the metal fibers is coated with a material other than metal.

9. The metal nonwoven fabric according to claim 1, wherein the metal fibers are composed of a core portion made of a metal and a shell portion disposed on the surface of the core portion and made of a metal other than the metal.

10. An electrode made using the nonwoven metal fabric according to claim 1.