Metal nonwoven fabric, electrode and joining material using same, and production method for metal nonwoven fabric

Abstract

This metal nonwoven fabric includes metal fibers. The compressive elastic modulus is no more than 4 MPa. When a rectangular piece of the metal nonwoven fabric that has a length of 300 mm, a width of 200 mm, and a thickness of 1 mm is hung such that the length direction coincides with the vertical direction, the percentage change in the length in the length direction of the metal nonwoven fabric is no more than 10%. The tensile strength is 0.01–4 MPa. When a rectangular piece of the metal nonwoven fabric that has a length of 50 mm, a width of 10 mm, and a thickness of 1 mm is placed on a horizontal surface, one end in the length direction of the metal nonwoven fabric is fixed to the horizontal surface, and the other end is pulled up such that the maximum curvature is at a center part in the length direction of the metal nonwoven fabric and the internal angle formed between the surface of a region at least 10 mm from the other end and the horizontal surface is 120 degrees, the metal non-woven fabric does not break.

Description

Metallic nonwoven fabric, electrode and bonding material using same, and method for manufacturing metallic nonwoven fabric

[0001] The present invention relates to a flexible metal nonwoven fabric, an electrode and a bonding material using the same, and a method for manufacturing the metal nonwoven fabric.

[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.) not found in conventional materials. Known conventional technologies relating to the use of metal fibers include those described in Patent Documents 1 and 2 and Non-Patent Document 1.

[0003] Patent Document 1 describes the use of a nonwoven fabric formed by entangling metal fibers by generating convection in a reaction solution in which metal fibers made of a ferromagnetic metal are formed, as an electrode. The document also describes that this electrode has improved flexibility and, when used in a battery, improved performance such as cycle characteristics.

[0004] Patent Document 2 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.

[0005] 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.

[0006] International Publication No. 2014 / 147885 Pamphlet US2019 / 0305322A1

[0007] F. Yang, et al. , Adv. Energy Mater, 2020, 10, 20011174.

[0008] Metallic materials with fine voids, such as porous metal bodies and metal foams (collectively referred to as "porous metal bodies"), have electrical conductivity and a large surface area, and are being considered for use as electrodes in fuel cells, water splitting electrolysis, carbon dioxide reduction, etc. For example, the techniques described in Patent Documents 1 and 2 and Non-Patent Document 1 utilize the fine diameter of metal fibers, which allows them to be used as electrodes with a large surface area (i.e., porous metal bodies).

[0009] Metal porous bodies are required to have flexibility and dimensional stability so that their physical and chemical properties can be exerted, but Patent Document 2 and Non-Patent Document 1 make no mention of flexibility. Furthermore, Patent Documents 1 and 2, and Non-Patent Document 1 make no mention of achieving both flexibility and dimensional stability. Furthermore, for industrial use, it is also required that the area of ​​metal porous bodies can be increased.

[0010] Therefore, the inventors conducted extensive research into the problems associated with the prior art and discovered that flexibility and dimensional stability can be achieved by producing a nonwoven fabric using specified metal fibers so as to have specified voids, leading to the present invention.

[0011] That is, the present invention provides a metal nonwoven fabric comprising metal fibers, which has a compressive modulus of elasticity of 4 MPa or less, and when the metal nonwoven fabric is formed into a rectangular shape of 300 mm length x 200 mm width x 1 mm thickness and is hung so that the length of the metal nonwoven fabric coincides with the vertical direction, the rate of change in length of the metal nonwoven fabric in the length direction is 10% or less.

[0012] The present invention also provides a metal nonwoven fabric comprising metal fibers, the metal nonwoven fabric having a compressive modulus of elasticity of 4 MPa or less and a tensile strength of 0.01 MPa to 4 MPa or less, wherein the metal nonwoven fabric is formed into a rectangular shape of 50 mm in length, 10 mm in width and 1 mm in thickness, and is placed on a horizontal surface with one end in the longitudinal direction of the metal nonwoven fabric fixed to the horizontal surface, and the other end is pulled up so that the central part in the longitudinal direction of the metal nonwoven fabric has the maximum curvature and the interior angle between a surface in a region at least 10 mm from the other end and the horizontal plane is 120 degrees, without breaking.

[0013] The present invention also provides a method for producing a metal nonwoven fabric, which comprises using an electrolyte containing a metal element source and precipitating metal fibers on a cathode by electrolytic reduction in a state in which an oily substance is present on the surface of the cathode; dispersing the metal fibers and an organic solvent by a thin film spinning method to obtain a slurry; and pouring the slurry into a mold to obtain a deposit of the metal fibers, which is then pressed and fired to obtain a metal nonwoven fabric.

[0014] Figures 1(a) to 1(c) are schematic diagrams showing a method for measuring the rate of change in the longitudinal length of the nonwoven metal fabric of the present invention. Figures 2(a) to 2(c) are schematic diagrams showing a method for measuring the folding endurance of the nonwoven metal fabric of the present invention.

[0015] 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.

[0016] 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, and oxide fibers. It may also contain substances in shapes other than fibers. 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.

[0017] 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.

[0018] 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 gives the metal nonwoven fabric flexibility and 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. "Identical" here 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.

[0019] Porous metal bodies have been considered for use as current collectors for various batteries, electrodes for power generation devices and electrolyzers, heat dissipation materials, or bonding materials, taking advantage of their electrical conductivity and high surface area. For example, when a porous metal body is installed in a cell as an electrode, it must be flexible enough to be attached to the cell frame without gaps so that liquids and gases can fully electrochemically react in the space within the porous metal body. If gaps are formed in the cell frame, unreacted solution or gas will leak through the gaps, reducing the reaction efficiency of the electrochemical reaction. Furthermore, in the field of electronic circuits, there is a demand for metal-to-metal bonding technology that is resistant to deterioration in electrical and thermal conductivity. In this technology, deterioration of electrical and thermal conductivity due to physical gaps at internal and external joints of electronic device substrates and electrical joints around power semiconductors is a major challenge, and flexibility of porous metal bodies is also required in this field.

[0020] On the other hand, porous metal bodies are also required to have dimensional stability when used as, for example, electrodes or bonding materials. For example, even if a porous metal body has flexibility when used as an electrode or bonding material, if the shape changes to such an extent that the intended performance cannot be exhibited, the porous metal body will become unusable. Even if the porous metal body has flexibility, it is undesirable for the porous metal body to change shape and thereby fail to obtain the high surface area and small pore diameter that are important functions of nonwoven fabrics.

[0021] In contrast, the metal nonwoven fabric of the present invention combines sufficient flexibility with dimensional stability that maintains a high surface area and small pore size. Therefore, when the metal nonwoven fabric is used as an electrode or electrode bonding material, the metal nonwoven fabric is easily attached to the cell and is less likely to deteriorate in electrical and thermal conductivity. Furthermore, even when used as a bonding material, the nonwoven fabric can absorb expansion caused by heat.

[0022] The metal nonwoven fabric of the present invention has flexibility as a whole. The flexibility of a metal nonwoven fabric can be determined by its compressive modulus, with a lower compressive modulus indicating a more flexible metal nonwoven fabric. The compressive modulus of the metal nonwoven fabric is preferably 4 MPa or less, more preferably 3.5 MPa or less, even more preferably 2 MPa or less, and even more preferably 1 MPa or less. Furthermore, from the viewpoint of imparting to the metal nonwoven fabric sufficient strength to form a nonwoven fabric with a large area of ​​300 mm length x 200 mm width, the compressive modulus is preferably 0.01 MPa or more.

[0023] The compressive modulus of a metal nonwoven fabric can be measured by the following method. First, a test piece measuring 5 mm long x 5 mm wide x 1 mm thick is cut out from a dry metal nonwoven fabric. If the thickness of the metal nonwoven fabric is less than 1 mm, multiple sheets of metal nonwoven fabric are stacked to adjust the thickness to 1 mm. On the other hand, if the thickness is greater than 1 mm, the metal nonwoven fabric is scraped with a file or the like to adjust the thickness to 1 mm. Next, a precision material testing machine (Instron Model 5848) or a small universal material testing machine (Instron Model 5565) is used to measure the modulus using the crosshead movement method. These two testing machines are used depending on the load level. When using a precision material testing machine, a 10 N capacity load cell is used for measurement. When using a small universal material testing machine, a 100 N capacity load cell is used for measurement. The temperature is room temperature (25°C) and the atmosphere is air. A 12.5 mm diameter indenter is pressed against the center of the test piece, and a compression test (0.5 mm / min) is performed in the thickness direction. The compressive modulus is determined based on the gradient of a linear approximation in a compression load-crosshead movement diagram after instrument compliance correction.

[0024] The compressive modulus of a metal nonwoven fabric is adjusted appropriately depending on the application of the metal nonwoven fabric. As will be described later, the compressive modulus of a metal nonwoven fabric can be controlled, for example, by adjusting the average fiber diameter of the metal fibers in the metal nonwoven fabric or the size of the voids within the metal nonwoven fabric. It can also be controlled by coating the surface of a molded metal nonwoven fabric with a metal species containing the metal by plating, sputtering, or the like.

[0025] The metal nonwoven fabric of the present invention has sufficient dimensional stability as a whole. In this specification, "dimensional stability" is defined by either of the following two indicators: i) when a metal nonwoven fabric formed into a rectangular shape of 300 mm length x 200 mm width x 1 mm thickness is hung so that the length direction of the metal nonwoven fabric coincides with the vertical direction, the rate of change in length of the metal nonwoven fabric in the longitudinal direction is 10% or less, or ii) when a metal nonwoven fabric having a tensile strength of 0.01 MPa to 4 MPa and formed into a rectangular shape of 50 mm length x 10 mm width x 1 mm thickness is placed on a horizontal surface and one end in the longitudinal direction of the metal nonwoven fabric is fixed to the horizontal surface, the metal nonwoven fabric does not break even when the other end is pulled up so that the central part in the longitudinal direction of the metal nonwoven fabric has the maximum curvature and the interior angle between the surface of a region at least 10 mm from the other end and the horizontal plane is 120 degrees.

[0026] One indicator of dimensional stability is i) the rate of change in the length of a metal nonwoven fabric in the longitudinal direction when the metal nonwoven fabric is formed into a rectangular shape measuring 300 mm in length, 200 mm in width, and 1 mm in thickness and hung so that the longitudinal direction is aligned with the vertical direction. The smaller the rate of change in dimension based on this measurement method, the higher the dimensional stability of the metal nonwoven fabric. Because the metal nonwoven fabric has dimensional stability, when the metal nonwoven fabric is installed in a cell for use as an electrode, thick and thin portions of the metal nonwoven fabric are less likely to occur. This makes it possible to suppress performance degradation of batteries, power generators, and electrolyzers due to thickness unevenness or thin portions, as well as mechanical degradation of the metal nonwoven fabric.

[0027] The dimensional stability of a metal nonwoven fabric can be measured using the method shown in Figures 1(a) to 1(c). Specifically, as shown in Figure 1(a), a dry metal nonwoven fabric 1 is first formed into a rectangular shape measuring 300 mm long x 200 mm wide x 1 mm thick. If the metal nonwoven fabric 1 itself lacks strength and cannot be fabricated to a size of 300 mm long x 200 mm wide x 1 mm thick, it is determined to have no dimensional stability. If the thickness of the metal nonwoven fabric 1 is greater than 1 mm, the metal nonwoven fabric 1 is filed down to a thickness of 1 mm. Next, as shown in Figure 1(b), the metal nonwoven fabric 1 is gripped at a position 5 mm inward from one horizontal edge and 5 mm inward from both vertical edges. Any clip 2 may be used for gripping. The metal nonwoven fabric 1 is lifted by the gripped portion and hung so that the longitudinal direction of the metal nonwoven fabric 1 coincides with the vertical direction. At this time, the distance between the lower edge of the metal nonwoven fabric 1 and the horizontal plane 3 is 50 mm, and the metal nonwoven fabric 1 is held in this state. As shown in Figure 1 (c), five minutes after gripping, the length W1 of the longitudinal side of the metal nonwoven fabric 1 is measured in this state. If the lengths of both longitudinal sides are different, the longer length is used. The value is calculated based on the following formula (1). The above measurement is performed in the same way for five pieces of metal nonwoven fabric 1, and the obtained values ​​are arithmetically averaged. This value is the rate of change in the longitudinal length of the metal nonwoven fabric 1. (Length W1 (mm) of the metal nonwoven fabric after hanging - 300 (mm) / 300 (mm)) x 100 (%) ... (1)

[0028] When the above-mentioned measurement method is adopted, from the viewpoint of imparting to the metal nonwoven fabric a strength sufficient to form a nonwoven fabric with a large area of ​​300 mm length x 200 mm width, it is preferable that the rate of change in the length of the metal nonwoven fabric in the longitudinal direction is 10% or less, more preferably 5% or less, and may be 3% or less, 1% or less, or even 0%.

[0029] Next, we will explain the other indicator of dimensional stability, ii) whether the tensile strength is 0.01 MPa or more and 4 MPa or less, and whether the metal nonwoven fabric formed into a rectangular shape of 50 mm length x 10 mm width x 1 mm thickness is placed on a horizontal surface, one end of the metal nonwoven fabric in the longitudinal direction is fixed to the horizontal surface, and the other end does not break even when pulled up so that the central part of the metal nonwoven fabric in the longitudinal direction has the maximum curvature and the inner angle between the surface of an area at least 10 mm from the other end and the horizontal plane is 120 degrees.

[0030] First, if the tensile strength of the metal nonwoven fabric is within a predetermined range, the metal nonwoven fabric will have sufficient strength, leading to less dimensional change. Specifically, from the viewpoint of imparting to the metal nonwoven fabric a strength sufficient to form a nonwoven fabric with a large area of ​​300 mm length x 200 mm width, the tensile strength of the metal nonwoven fabric is preferably 0.01 MPa or more. Furthermore, from the viewpoint of improving the flexibility of the metal nonwoven fabric, the tensile strength is preferably 4 MPa or less, more preferably 2 MPa or less, even more preferably 1 MPa or less, even more preferably 0.4 MPa or less, and even more preferably 0.25 MPa or less.

[0031] The tensile strength of metal nonwoven fabric can be measured using the following method. First, a test piece measuring 50 mm long x 10 mm wide x 1 mm thick is cut out from dry metal nonwoven fabric. If the thickness of the metal nonwoven fabric is less than 1 mm, multiple sheets of metal nonwoven fabric are stacked to adjust the thickness to 1 mm. On the other hand, if the thickness is greater than 1 mm, the metal nonwoven fabric is scraped with a file or other tool to adjust the thickness to 1 mm. Measurement is performed using a small universal testing machine (Instron Model 5565) using the crosshead movement method. The temperature is room temperature (25°C) and the atmosphere is air. A tensile test (0.5 mm / min) is performed in the vertical direction. The maximum load is read from the tensile load-crosshead movement diagram after instrument compliance correction, and this value is divided by the cross-sectional area of ​​the test piece before the tensile test to determine the tensile strength. For convenience, a metal nonwoven fabric that is too weak to be attached to the above-mentioned device is considered to have a tensile strength of less than 0.01 MPa.

[0032] Furthermore, the metal nonwoven fabric must be able to withstand the bending test described above by placing a rectangular metal nonwoven fabric measuring 50 mm in length, 10 mm in width, and 1 mm in thickness on a horizontal surface, fixing one end of the metal nonwoven fabric in the longitudinal direction to the horizontal surface, and then pulling up the other end so that the central portion of the metal nonwoven fabric in the longitudinal direction has the maximum curvature and the interior angle between the surface of a region at least 10 mm from the other end and the horizontal plane is 120 degrees. This requires the fabric to withstand the bending test without breaking (i.e., folding endurance). In addition to having the tensile strength described above, not breaking in the above test results in excellent dimensional stability. The dimensional stability of the metal nonwoven fabric makes it less likely for thick and thin portions to form on the metal nonwoven fabric when installed in a cell for use as an electrode. This reduces the likelihood of performance degradation of batteries, power generators, and electrolyzers due to thickness variations or thin portions, as well as mechanical degradation of the metal nonwoven fabric.

[0033] The folding endurance of the above-mentioned metal nonwoven fabric can be measured using the method shown in Figures 2(a) to 2(c). Specifically, as shown in Figure 2(a), first, a dry metal nonwoven fabric 1 is formed into a rectangular shape measuring 50 mm long x 10 mm wide x 1 mm thick. If the size of the metal nonwoven fabric 1 is less than 50 mm long x 10 mm wide, a rectangular metal nonwoven fabric 1 is formed as large as possible with the same aspect ratio. If the thickness of the metal nonwoven fabric 1 is less than the predetermined thickness, multiple sheets of metal nonwoven fabric 1 are stacked to adjust the thickness to the predetermined thickness. On the other hand, if the thickness is greater than the predetermined thickness, the metal nonwoven fabric 1 is filed with a file or the like to adjust the thickness to the predetermined thickness. Next, as shown in Figure 2(b), the metal nonwoven fabric 1 is placed on a horizontal surface 3. An end portion 1a of the metal nonwoven fabric 1, extending 5 mm inward from one longitudinal edge, is fixed to the horizontal surface 3 using an arbitrary fixture 4 or the like. As shown in Figure 2(c), the fixed end 1a is used as a fulcrum, and the metal nonwoven fabric 1 is gripped with a clip 2 or tweezers at a position 3 mm inward from the other end 1b. The other end 1b is then pulled up at a rate of 10° / 10 seconds. The pulling position of the other end 1b is adjusted so that the central portion 1c of the vertical plane of the metal nonwoven fabric 1 has the maximum curvature, and the angle θ between the plane of a region D at least 10 mm from the other end 1b and the horizontal plane 3 is 120°. The gripping margin of the metal nonwoven fabric 1 is included in the "10 mm" measurement. The metal nonwoven fabric 1 is then released from its fixed position, and the location is visually inspected for fractures. "Fracture" here refers to a state in which the metal nonwoven fabric 1 is torn or cracked. The above-described measurement is performed on 10 sheets of metal nonwoven fabric 1. If breaks or cracks occur in four or more of the ten sheets of metal nonwoven fabric 1, the metal nonwoven fabric 1 is judged to have no folding endurance. Even if breaks or cracks occur in three or fewer of the ten sheets of metal nonwoven fabric 1, the metal nonwoven fabric 1 is judged to have folding endurance. Note that if the vertical length of the metal nonwoven fabric 1 is less than 50 mm, the "10 mm" in the above-mentioned "surface in a region at least 10 mm from the other end 1b" should be interpreted as a length proportional to the vertical length of the metal nonwoven fabric 1.

[0034] The longitudinal length change rate, tensile strength, and folding endurance of a metal nonwoven fabric are adjusted appropriately depending on the application of the metal nonwoven fabric. The longitudinal length change rate, tensile strength, and folding endurance of a metal nonwoven fabric can be controlled, for example, by adjusting the average fiber diameter and average length of the metal fibers in the metal nonwoven fabric or the size of the voids within the metal nonwoven fabric. They can also be controlled by coating the surface of a molded metal nonwoven fabric with a metal species containing the metal by plating, sputtering, or the like.

[0035] In a preferred embodiment of the metal nonwoven fabric, both the front and back surfaces of the metal nonwoven fabric are preferably substantially flat. In other words, it is preferable that the front and back surfaces of the metal nonwoven fabric have high smoothness. While the smoothness of a metal nonwoven fabric can be measured using a laser microscope or the like, here, it is defined by the mean deviation (SMD) of the surface roughness being equal to or less than a predetermined value using a roughness / friction tester KES-SESRU manufactured by Kato Tech Co., Ltd. The smaller the surface roughness, the smoother the metal nonwoven fabric. High smoothness of the metal nonwoven fabric reduces the frequency of gaps occurring when attaching the metal nonwoven fabric to a cell frame for use as an electrode. Furthermore, since thick and thin portions of the metal nonwoven fabric are less likely to occur, performance degradation of batteries, power generators, and electrolyzers due to thickness unevenness or thin portions, as well as mechanical degradation of the metal nonwoven fabric, can be suppressed.

[0036] The mean deviation of surface roughness (SMD) is measured according to the measurement principle described in the following book: "Standardization and Analysis of Texture Evaluation," 2nd Edition, by Toshio Kawabata, published by the Texture Measurement and Standardization Research Committee of the Japan Textile Machinery Association, July 10, 1980. Specifically, a dry metal nonwoven fabric is first formed into a rectangular shape measuring 50 mm long and 10 mm wide to obtain a sample. If the size of the metal nonwoven fabric is less than 50 mm long and 10 mm wide, the largest possible rectangular shape is formed. The metal nonwoven fabric is attached to a smooth, flat metal test stand. The contactor is a 0.5 mm diameter piano wire bent into a U-shape with a width of 5 mm. The contact surface of the contactor is pressed against the metal nonwoven fabric by a spring. The force is 10 g force. The sensitivity is H. The metal nonwoven fabric is moved horizontally for 2.7 cm at a constant speed of 0.1 cm / sec. The measured value of the mean deviation of surface roughness (SMD) is expressed as an SMD value. Note that in this method, the load-applied sensor (contact) is directly in contact with the sample for measurement. Therefore, it should be noted that the obtained mean deviation of surface roughness (SMD) does not directly indicate the degree of unevenness of the sample surface when no load is applied.

[0037] When the above-mentioned measurement method is adopted, from the viewpoint of suppressing deterioration of the performance as an electrode and suppressing mechanical deterioration of the metal nonwoven fabric, the mean deviation SMD of the surface roughness of the metal nonwoven fabric is preferably 3 μm or less, more preferably 2 μm or less, and even more preferably 1.6 μm or less. The smaller the mean deviation SMD of the surface roughness, the better, but it may be 1 μm or more.

[0038] The level of the mean deviation of surface roughness SMD of a metal nonwoven fabric is adjusted appropriately depending on the application of the metal nonwoven fabric. The mean deviation of surface roughness SMD of a metal nonwoven fabric can be controlled by dispersing the slurry under predetermined conditions or by pressurizing the deposit in the manufacturing method described below.

[0039] As long as the metal nonwoven fabric has sufficient flexibility as a whole, it may be a three-layer or more 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 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 Z 8801) 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.

[0040] From the viewpoint of achieving both sufficient flexibility and dimensional stability in the metal nonwoven fabric, the average fiber diameter of the metal fibers is preferably within a predetermined range. Specifically, from the viewpoint of imparting dimensional stability to the metal nonwoven fabric sufficient to form a nonwoven fabric with a large area of ​​300 mm length x 200 mm width, the average fiber diameter of the metal fibers is preferably 20 nm or more, more preferably 30 nm or more, even more preferably 90 nm or more, even more preferably 120 nm or more, even more preferably 500 nm or more, and particularly preferably 700 nm or more. Furthermore, from the viewpoint of improving the flexibility of the metal nonwoven fabric, the average fiber diameter of the metal fibers is preferably 10 μm or less, more preferably 6 μm or less, even more preferably 3 μm or less, and even more preferably 2.5 μm or less.

[0041] From the viewpoint of achieving both sufficient flexibility and dimensional stability in the metal nonwoven fabric, the average length of the metal fibers is preferably within a predetermined range. Specifically, from the viewpoint of imparting dimensional stability to the metal nonwoven fabric sufficient to form a nonwoven fabric with a large area of ​​300 mm length x 200 mm width, the average length of the metal fibers is preferably 3 μm or more, more preferably 5 μm or more, even more preferably 10 μm or more, even more preferably 30 μm or more, and even more preferably 50 μm or more. Furthermore, from the viewpoint of improving dispersibility in the slurry in the manufacturing method described below to prevent unevenness and improving the flexibility of the metal nonwoven fabric, the average length of the metal fibers is preferably 5000 μm or less, more preferably 2000 μm or less, even more preferably 500 μm or less, even more preferably 250 μm or less, and even more preferably 150 μm or less.

[0042] 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.

[0043] As described above, the metal fibers in a metal nonwoven fabric are preferably very thin and long. Having such average fiber diameter and length improves the flexibility of the metal nonwoven fabric and also increases the dimensional stability between the metal fibers, allowing the formation of a large-area nonwoven fabric measuring 300 mm long x 200 mm wide. In addition, these properties of the metal fibers can be easily controlled, and the handling of the metal fibers can be improved.

[0044] From the viewpoint of improving the flexibility of the metal nonwoven fabric and imparting to the metal nonwoven fabric dimensional stability sufficient to form a nonwoven fabric with a large area of ​​300 mm length x 200 mm width, the aspect ratio of the metal fibers (length of metal fiber [m] / fiber diameter of metal fiber [m]) is preferably 5 or more, and more preferably 20 or more. From the same viewpoint, the aspect ratio is preferably 5000 or less, more preferably 3000 or less, even more preferably 1500 or less, even more preferably 800 or less, even more preferably 200 or less, and particularly preferably 100 or less.

[0045] 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 500x 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 arithmetic mean is then calculated to obtain the average fiber diameter of the metal fibers.

[0046] 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.

[0047] The metal fibers in a metal nonwoven fabric may have a substantially uniform average fiber diameter along their entire length, or may have a beaded average fiber diameter that is not uniform. It is preferable that at least one end of the metal fibers be 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 not only in terms of exhibiting mechanical strength but also in terms of maintaining electrical and thermal conductivity. Furthermore, metal fibers with at least one end tapered 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.

[0048] 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.

[0049] 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 500x 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.

[0050] 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 to improve flexibility and 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.

[0051] 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 such 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.

[0052] The surfaces of the metal fibers in the metal nonwoven fabric may be coated with a substance other than metal. The term "coated" as used herein 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. The metal fibers may be coated in either one of these states or both of these states. Examples of the nonmetallic substance that may be coated on the metal fibers in the metal nonwoven fabric 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 the metal nonwoven fabric is used as an electrode catalyst, 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 another substance 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.

[0053] The metal fibers in the metal nonwoven fabric may be composed of 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 laminated with 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 that constitute the core include copper, silver, gold, nickel, lead, palladium, platinum, cobalt, tin, iron, zinc, or bismuth, or alloys containing these metals. The metal that constitutes the shell is not particularly limited as long as it is a metal different from the metal that constitutes the core, and examples include copper, silver, nickel, tin, zinc, lead, iron, cobalt, platinum, gold, and palladium. The method for producing metal fibers comprising a core portion and a shell portion is not particularly limited. Examples of suitable methods include depositing metal fibers using the method described below, followed by electroplating in an electrolytic bath containing a coating material, displacement plating, electroless plating, depositing a catalyst used in these plating methods on metal fibers, and then plating the target material onto them, electrophoresis, 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 deposition method, an atomic layer deposition method (ALD method), an ink method, a fine particle coating method, or a dry coating method. The order in which metal fibers comprising a core portion and a shell portion are produced is not particularly limited. For example, the metal fibers may be produced by the above-described method before producing a metal nonwoven fabric, or a metal nonwoven fabric made of a metal that will become the core portion may be produced, and then a shell portion may be formed on the surface of the metal fibers in the metal nonwoven fabric by the above-described method. From the viewpoint of efficiently forming a shell portion on the surface of the core portion, the latter method is preferred. Furthermore, the coating of the metal nonwoven fabric will be mentioned later, and the coating can be carried out by the same methods as those described above.

[0054] The metal fibers in metal nonwoven fabrics preferably have a polycrystalline structure in which multiple crystals are connected along the extension direction 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 lower temperatures and more moderate conditions. 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 easily oxidized 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, increasing the contact area between the metal fibers and improving mechanical strength, electrical conductivity, and thermal conductivity. Note that the "extension direction" of the metal fibers refers to the tangential direction when the metal fibers have curved portions.

[0055] 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. As mentioned above, the average fiber diameter of the metal fibers is preferably 20 nm or more and 10 μm or less, and therefore 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 to exhibit mechanical strength, electrical conductivity, and thermal conductivity. To further enhance this advantage, the X / Y value is preferably 3 or less, and even more preferably 2 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 (instead of calculating X / Y from the average X and Y values, the X / Y values ​​at each location are averaged), 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).

[0056] 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.

[0057] When the value of X / Y is 4 or less, there are no restrictions on the values ​​of X and Y, but the value of X itself is preferably 10 μm or less, more preferably 2 μm or less, and even more preferably 500 nm or less, from the viewpoint of being able to lower the fusion temperature of the metal fiber. From the same viewpoint, the value of X itself is preferably 5 nm or more, and more preferably 10 nm or more. From the same viewpoint, the value of Y itself is preferably 3 μm or less, more preferably 1 μm or less, and even more preferably 400 nm or less. From the same viewpoint, the value of Y itself is preferably 5 nm or more, and more preferably 10 nm or more.

[0058] 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).

[0059] 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).

[0060] 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.

[0061] 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).

[0062] 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).

[0063] 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.

[0064] 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 equation (2): r = (C × C) / (8 × h) + h / 2 (2) (In formula (2), 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 the 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.

[0065] Metal nonwoven fabrics may contain particles having shapes other than metal fibers. However, since the metal nonwoven fabric is manufactured based on the idea of ​​controlling the gaps in the metal nonwoven fabric by randomly depositing fibers, it is preferable that particles having shapes other than fibers are present as little as possible in the metal nonwoven fabric in order to improve the liquid permeability of the metal nonwoven fabric even when subjected to deformation such as bending or stretching. When the proportion of particles having shapes other than fibers in a metal nonwoven fabric is defined as the "irregularity rate," the irregularity rate is preferably 50% or less, more preferably 40% or less, even more preferably 30% or less, even more preferably 15% or less, and particularly preferably 8% or less. By manufacturing a metal nonwoven fabric using the manufacturing method described below, it is possible to easily achieve an irregularity rate of 50% or less. The irregularity rate can be determined by observing the sample to be measured using an SEM in a field of view 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 ​​irregularly shaped particles / area of ​​the metal nonwoven fabric]. "Irregular shapes" refers to shapes other than fibers.

[0066] From the viewpoint of achieving both sufficient flexibility and dimensional stability in a metal nonwoven fabric, it is preferable that the void distribution peak top diameter be within a predetermined range. Specifically, from the viewpoint of imparting to the metal nonwoven fabric dimensional stability sufficient to form a nonwoven fabric with a large area of ​​300 mm length x 200 mm width, the void distribution peak top diameter of the metal nonwoven fabric measured by mercury porosimetry is preferably 50 μm or less, more preferably 30 μm or less, even more preferably 25 μm or less, and particularly preferably 20 μm or less. Furthermore, from the viewpoint of improving the flexibility of the metal nonwoven fabric, the void distribution peak top diameter is preferably 0.01 μm or more, more preferably 0.02 μm or more, even more preferably 0.05 μm or more, even more preferably 0.1 μm or more, even more preferably 1 μm or more, and particularly preferably 8 μm or more. In addition, 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 peak with the highest height falls within the above-mentioned range, as this makes the above-mentioned advantages even more pronounced.

[0067] 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.

[0068] The porosity of a metal nonwoven fabric can be calculated as the volume of the void space present within 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 metal constituting the metal nonwoven fabric. The volume V3 of the void space present within the metal nonwoven fabric is calculated as the percentage of the volume V3 of the void space present within the metal nonwoven fabric relative to the volume V1 of the metal nonwoven fabric, and this is the porosity. From the viewpoint of increasing the amount of liquid that permeates the metal nonwoven fabric, the porosity 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 dimensional stability of the metal nonwoven fabric, the porosity is preferably 99% or less.

[0069] 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.

[0070] As described above, the flexibility and dimensional stability of metal nonwoven fabrics can be controlled by the size of the voids within the metal nonwoven fabric, the average fiber diameter and average length of the metal fibers, and the metal or non-metal coating on the metal nonwoven fabric. For example, if a metal nonwoven fabric has excessively small voids, the compressive modulus of the metal nonwoven fabric increases. That is, the metal nonwoven fabric becomes stiff. For example, if a metal nonwoven fabric has excessively large voids, the compressive modulus of the metal nonwoven fabric decreases, improving flexibility, but the bonding strength between the metal fibers in the metal nonwoven fabric decreases, resulting in reduced dimensional stability. Alternatively, if the average fiber diameter of the metal fibers is excessively large, the compressive modulus of the metal nonwoven fabric increases. For example, if the average fiber diameter of the metal fibers is excessively small, the bonding strength between the metal fibers in the metal nonwoven fabric decreases, resulting in reduced dimensional stability. Alternatively, if the average length of the metal fibers is excessively short, the bonding strength between the metal fibers in the metal nonwoven fabric decreases, resulting in reduced dimensional stability. When a metal nonwoven fabric is coated with a metal or nonmetallic material, the compressive modulus is improved and the bonding strength between the metal fibers is increased, thereby improving dimensional stability. On the other hand, the metal nonwoven fabric of the present invention has improved flexibility, so that when the metal nonwoven fabric is pressed against a surface, even if the surface has an uneven shape, the metal nonwoven fabric can deform to fit the uneven shape and cover the surface without any gaps, resulting in a very tight adhesion. As a result, gaps larger than the pores in the metal nonwoven fabric are less likely to form, thereby suppressing deterioration of electrical and thermal conductivity. Furthermore, because the metal nonwoven fabric of the present invention has improved strength, when the metal nonwoven fabric is installed in a cell for use as an electrode, thick and thin portions of the metal nonwoven fabric are less likely to occur, thereby suppressing performance degradation of batteries, power generators, and electrolyzers due to thickness unevenness or thin portions, and mechanical degradation of the metal nonwoven fabric.

[0071] From the viewpoint of improving the reactivity with a liquid when the metal nonwoven fabric is used as an electrode, for example, and from the viewpoint of satisfying practicality as an electrode, the average thickness of the metal nonwoven fabric is preferably 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 depends on the application and cannot be generally stated, 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.

[0072] 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 size of 2 cm length x 2 cm width 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 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 on the metal nonwoven fabric are measured, and the arithmetic average of these values ​​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.

[0073] From the viewpoint of improving the reactivity with a liquid when the metal nonwoven fabric is used as an electrode, for example, and from the viewpoint of satisfying practicality as an electrode, the metal nonwoven fabric has a specific surface area of ​​0.02 m ２ / g or more, and ２ / g or more, and 0.2m ２ / g or more, and more preferably 0.25m ２ From the same viewpoint, it is more preferable that the specific surface area of ​​the metal nonwoven fabric is 22 m / g or more. ２ / g or less, and２ / g or less is more preferable, and ２ / g or less is more preferable, and 2m ２ / g or less is more preferable, and ２ It is even more preferable that the SiO2 content is 1 / g or less.

[0074] 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 Japan Bell. 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.

[0075] 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 fabrics.

[0076] From the viewpoint of improving the reactivity with a liquid when the metal nonwoven fabric is used as an electrode, for example, and from the viewpoint of satisfying practicality as an electrode, the metal nonwoven fabric has an apparent density of 0.01 g / cm ３ It is preferable that the density is 0.05 g / cm or more. ３ More preferably, it is 0.1 g / cm or more. ３ From the same viewpoint, it is more preferable that the apparent density of the metal nonwoven fabric is 2.0 g / cm or more. ３ It is preferable that the density is 1.7 g / cm or less. ３ More preferably, it is 1.4 g / cm or less. ３ More preferably, it is 1.0 g / cm or less. ３ More preferably, it is 0.9 g / cm or less. ３It is even more preferable that the 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.

[0077] 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.

[0078] 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.

[0079] 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 a metal nonwoven fabric is produced, the steps include dispersing the precipitated metal fibers in an organic solvent to obtain a slurry, pouring the slurry into a mold to allow it to settle, removing the supernatant liquid, applying pressure, drying, and then reducing the mixture.

[0080] 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.

[0081] 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.

[0082] 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 compound 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 having preferably 6 or more, more preferably 10 or more, and even more preferably 11 or more, carbon atoms, and preferably 25 or less, more preferably 22 or less, and even more preferably 20 or less.

[0083] 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.

[0084] Once the metal fibers have precipitated, they are recovered from the cathode surface. Next, the metal fibers are dispersed in an organic solvent to obtain a slurry. Examples of the organic solvent include aliphatic alcohols. Examples of aliphatic alcohols include 2-propanol. By dispersing the metal fibers to obtain a slurry, the metal fibers are dispersed throughout the slurry, reducing the likelihood of uneven distribution of the metal fibers after pouring into a mold. As a result, unevenness within the metal nonwoven fabric is reduced, reducing the likelihood of localized areas of weak mechanical strength. Furthermore, the flexibility of the metal nonwoven fabric can be uniformly achieved. Furthermore, the smoothness of the desired metal nonwoven fabric can be improved. The inventors discovered that the surface shape of the resulting metal nonwoven fabric changes depending on the dispersion conditions. Based on this knowledge, they discovered that by dispersing the metal nonwoven fabric under specified conditions to obtain a slurry, a metal nonwoven fabric with a specified surface roughness mean deviation (SMD) can be obtained. Unlike the technology described in Patent Document 1, this method not only provides a low level of surface irregularity but also produces a more uniform metal nonwoven fabric with, for example, an apparent density and porosity within a specified range.

[0085] For dispersion, it is preferable to adopt a method that effectively disperses the metal fibers, as this improves the dimensional stability of the metal nonwoven fabric, provides a desired level of flexibility, and facilitates the production of a metal nonwoven fabric with low surface irregularities and high smoothness. A thin film swirl dispersion method is advantageous. With this dispersion method, the slurry in the container is pressed against the container wall by centrifugal force, causing the slurry droplets to move along the wall. The force resulting from the droplet movement and the centrifugal force then shear, effectively dispersing the metal fibers in the slurry. For dispersion using the thin film swirl method, for example, a thin film swirl high-speed mixer, FILMICS (registered trademark) manufactured by PRIMIX Corporation, can be used. The various conditions for dispersion using the thin film swirl method, such as the peripheral speed and processing time, should be such that the metal fibers can be effectively dispersed, as described in the examples below. Furthermore, ultrasonic dispersion treatment can be used to more effectively disperse the metal fibers.

[0086] Whichever method is adopted, it is preferable to disperse the metal fibers from the viewpoints of obtaining a metal nonwoven fabric with high smoothness, preventing unevenness in the metal nonwoven fabric that causes localized weak mechanical strength, and achieving uniform and appropriate flexibility in the metal nonwoven fabric as a whole. It is also preferable to disperse the metal fibers so that the average length of the dispersed metal fibers is 3 μm or more and 5000 μm or less.

[0087] Next, the slurry is allowed to stand, allowing the metal fibers to settle, producing a metal nonwoven fabric. Specifically, the dispersed slurry is poured into a water-impermeable mold, and the mold is allowed to stand, yielding a deposit of metal fibers. The mold dimensions may be approximately the same as or larger than the dimensions of the target 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 flexibility, dimensional stability, thickness, liquid permeability, and 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 directly. After drying, the filter can be removed to produce the metal nonwoven fabric. 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-described 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 the metal that constitutes the metal nonwoven fabric or a metal different from the metal that constitutes the metal nonwoven fabric.

[0088] Once the metal fiber deposit is obtained, 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.

[0089] After removing the supernatant liquid from the mold, the deposit remaining in the mold is dried under atmospheric pressure, an inert gas atmosphere, or a vacuum atmosphere. Drying can be carried out for, for example, 1 hour to 24 hours, more preferably 4 hours to 16 hours. In 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.

[0090] The deposit is then pressed and calcined. Pressing and calcination may be performed simultaneously or separately. In the latter case, pressing and calcination may be performed in that order, or calcination and pressing may be performed in that order. When pressing and calcination are performed separately, the deposit may be calcined in an argon, nitrogen, or hydrogen-containing atmosphere after or before pressing the deposit. This fuses the intersections of the metal fibers in the deposit, resulting in a metal nonwoven fabric. If it is necessary to strongly reduce the surfaces of the metal fibers during fusion, the deposit may be 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 in a hydrogen-containing atmosphere to reduce the surfaces of the metal fibers. Furthermore, when metal fibers are obtained by the above-mentioned electrolytic method, reducing and calcining the deposit also has the advantage of making at least some of the metal fibers in the deposit more easily fused. The time for calcining the deposit is preferably 3 minutes or more, more preferably 30 minutes or more, from the viewpoint of sufficiently and efficiently reducing the surface oxides and fusing the metal fibers together. From the same viewpoint, the time for calcining is preferably 24 hours or less, more preferably 5 hours or less. The temperature for calcining the deposit is preferably 800°C or less, more preferably 150°C or more and 500°C or less, from the viewpoint of efficiently reducing the metal and achieving adequate fusing between the metal fibers.

[0091] In addition, the metal nonwoven fabric is pressed in the thickness direction before or after calcination. Pressurization can further increase the smoothness of the metal nonwoven fabric. By adjusting the pressure, time, temperature, etc., when pressing the metal nonwoven fabric in the thickness direction, a metal nonwoven fabric having the desired flexibility, dimensional stability, thickness, void size, and porosity can be obtained. When pressing the metal nonwoven fabric in the thickness direction, for example, the metal nonwoven fabric may be removed from the mold, metal plates (spacers) having the desired thickness may be placed at the four corners of the metal nonwoven fabric, and pressure may be applied from above the metal plates. The pressure when pressing the metal nonwoven fabric is not particularly limited, as long as a metal nonwoven fabric having the desired flexibility, dimensional stability, thickness, void size, and porosity can be obtained. For example, from the viewpoint of successfully obtaining a metal nonwoven fabric of the desired size, the pressure is preferably 0.01 MPa or more, more preferably 1 MPa or more. From the same viewpoint, the pressure is preferably 50 MPa or less, more preferably 30 MPa or less, and even more preferably 20 MPa or less. From the viewpoint of obtaining a metal nonwoven fabric having the desired flexibility, dimensional stability, thickness, void size, and porosity within the metal nonwoven fabric, the time for pressing the metal nonwoven fabric is preferably 3 minutes or more, more preferably 10 minutes or more, and even more preferably 20 minutes or more. From the same viewpoint, the time is preferably 5 hours or less, more preferably 3 hours or less, and even more preferably 2 hours or less. When pressing the deposit, the deposit may be heated. Alternatively, the deposit may be pressed without heating. When heating the deposit, the temperature is preferably 500°C or less, and even more preferably 350°C or less, from the viewpoint of preventing excessive fusion between the metal fibers.

[0092] When pressing and firing of the deposit are carried out simultaneously, the pressing and firing times may be the same or different, and the pressing and firing temperatures may be the same or different.

[0093] The metal nonwoven fabric obtained in this manner may be cut into a desired shape. Alternatively, a plurality of the obtained metal nonwoven fabrics may be laminated to form a metal nonwoven fabric having a laminated structure. In addition to the heat treatment described above, after forming the metal nonwoven fabric, it is also possible to use it as an electrode and then metal-plat the entire surface of the metal nonwoven fabric. This allows the surface of the metal nonwoven fabric to be coated with a different type of metal, or to be plated with the same type of metal, thereby improving the mechanical strength of the metal nonwoven fabric and controlling the size and porosity of the voids within the metal nonwoven fabric.

[0094] By producing a metal nonwoven fabric using the above method, the metal nonwoven fabric as a whole can achieve both sufficient flexibility and dimensional stability. Furthermore, the above method makes it easy to produce a large-area metal nonwoven fabric. For example, when the metal nonwoven fabric is rectangular, a large-area nonwoven fabric can be obtained with longitudinal and lateral dimensions each independently of each other of preferably 50 mm or more, more preferably 70 mm or more, and even more preferably 100 mm or more.

[0095] 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.

[0096] The metal nonwoven fabric of the present invention obtained by the above method can be used as a functional material by utilizing the flexibility and dimensional stability of the metal nonwoven fabric and the physical and chemical properties of the metal fibers. Examples of such functional materials include electrodes, bonding materials, and thermal expansion inhibitors. When the metal nonwoven fabric of the present invention is used as an electrode, for example, the metal nonwoven fabric can be easily attached to a cell, and electrical and thermal conductivity are less likely to deteriorate. Furthermore, the nonwoven fabric can absorb thermal expansion. Furthermore, due to the dimensional stability of the metal nonwoven fabric, thick and thin portions of the metal nonwoven fabric are less likely to occur, thereby suppressing performance degradation of batteries, power generators, and electrolyzers due to thickness unevenness or thin portions, and deterioration of the metal nonwoven fabric itself.

[0097] Furthermore, when the metal nonwoven fabric of the present invention is used as a bonding material, for example, it can be used as a filler for the bonding surface to prevent a decrease in the electrical and thermal conduction efficiency at the joints of housings or the connections of electronic circuits (e.g., via connections, bump connections, interlayer connections, and direct connections between copper metals without gold or tin). At the joints of electronic circuits, more effective connections can be achieved by using metal nonwoven fabrics containing fibers whose surfaces are further coated with a metal or alloy having a lower melting point than the metal constituting the metal fibers, or composite metal nonwoven fabrics formed by mixing metal nonwoven fabrics containing metal fibers with fine particles made of a metal or alloy having a lower melting point than the metal constituting the metal fibers. Heating these metal nonwoven fabrics at a temperature above the melting point of the metal used for the coating melts the metal or its alloy in the voids within the metal nonwoven fabric, thereby increasing the bonding area between the metal fibers. As a result, the electrical reliability of the joints and the heat dissipation effect can be improved.

[0098] Furthermore, when the metal nonwoven fabric of the present invention is used, for example, as a thermal expansion inhibitor, by sandwiching the metal nonwoven fabric between electronic circuits, the flexibility of the metal nonwoven fabric can be utilized, and the metal nonwoven fabric can absorb expansion caused by heat.

[0099] Although the present invention has been described above based on the preferred embodiments, the present invention is not limited to the above embodiments.

[0100] The above-described embodiments of the present invention encompass the following technical concepts: [1] A metal nonwoven fabric comprising metal fibers, which has a compressive modulus of 4 MPa or less, and when the metal nonwoven fabric is formed into a rectangular shape of 300 mm length x 200 mm width x 1 mm thickness and is hung so that the length of the metal nonwoven fabric coincides with the vertical direction, the rate of change in length of the metal nonwoven fabric in the length direction is 10% or less. [2] A metal nonwoven fabric comprising metal fibers, the metal nonwoven fabric having a compressive modulus of 4 MPa or less and a tensile strength of 0.01 MPa to 4 MPa or less, wherein the metal nonwoven fabric is formed into a rectangular shape of 50 mm length x 10 mm width x 1 mm thickness, and is placed on a horizontal surface with one end of the metal nonwoven fabric in the longitudinal direction fixed to the horizontal surface, and the other end is pulled up so that the central part of the metal nonwoven fabric in the longitudinal direction has the maximum curvature and the interior angle between the surface of a region at least 10 mm from the other end and the horizontal plane is 120 degrees, without breaking.

[0101] [3] The metal nonwoven fabric according to [1] or [2], wherein the average fiber diameter of the metal fibers is 20 nm or more and 10 μm or less. [4] The metal nonwoven fabric according to any one of [1] to [3], wherein the void distribution peak top diameter measured by mercury porosimetry is 50 μm or less. [5] The metal nonwoven fabric according to any one of [1] to [4], wherein, for the 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. [6] The metal nonwoven fabric according to any one of [1] to [5], wherein the average length of the metal fibers is 3 μm or more and 5,000 μm or less. [7] The metal nonwoven fabric according to any one of [1] to [6], wherein the average thickness of the metal nonwoven fabric is 3 μm or more.

[0102] [8] Specific surface area is 0.02 m ２ / g or more 22m ２ / g or less. [9] The metal nonwoven fabric according to any one of [1] to [7], 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.

[10] The metal nonwoven fabric according to any one of [1] to [9], wherein the surfaces of the metal fibers are coated with a material other than metal.

[11] The metal nonwoven fabric according to any one of [1] to

[10] , 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.

[12] The metal nonwoven fabric according to any one of [1] to

[11] , wherein the mean deviation SMD of the surface roughness measured using KES-SESRU manufactured by Kato Tech Co., Ltd. in accordance with "Standardization and Analysis of Texture Evaluation," 2nd Edition, by Toshio Kawabata, published by the Texture Measurement and Standardization Research Committee of the Japan Textile Machinery Association on July 10, 1980, is 3 μm or less.

[0103]

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

[12] .

[14] A bonding material made using the metal nonwoven fabric according to any one of [1] to

[12] .

[15] A method for producing a metal nonwoven fabric, comprising: using an electrolyte containing a metal element source and precipitating metal fibers on a cathode by electrolytic reduction in a state in which an oily substance is present on the surface of the cathode; dispersing the metal fibers and an organic solvent by a thin film swirl method to obtain a slurry; and pouring the slurry into a mold to obtain a deposit of the metal fibers, and pressing and firing the resulting metal fiber deposit to obtain the metal nonwoven fabric.

[0104] 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."

[0105] 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 application 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 0°C. In this way, fibrous copper was electrodeposited on the surface of the cathode. The deposited fibrous copper, which had accumulated on the electrode with a low degree of agglomeration, was peeled off and dispersed in 2-propanol to form a slurry. This process was repeated until the required amount of fibrous copper was obtained. Dispersion was carried out using a Filmix® 56-L thin-film gyratory high-speed mixer manufactured by Primix Corporation. Dispersion conditions were a peripheral speed of 6 m / s and a processing time of 600 seconds. 234 mL of the slurry (fibrous copper concentration 100 g / L) was poured into a mold (300 mm length x 200 mm width x 15 mm height) with a Ti plate as the bottom plate, and the mold was left to stand at room temperature (25°C) for 1 hour to allow the precipitate to settle. 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. Next, a hydrogen-containing atmosphere (3 vol% H ２ Remainder N ２The deposit was calcined under a vacuum without pressure to obtain a metal nonwoven fabric. Calcination was carried out at 350°C for 3 hours. After removing the resulting metal nonwoven fabric, it was placed in the center of a 308mm x 208mm aluminum plate. Copper pieces measuring 20mm x 5mm x 1mm were placed at the four corners of the aluminum plate, and another 308mm x 208mm aluminum plate was placed on top of the copper pieces. The reduced metal nonwoven fabric was pressed against the aluminum plate, with the copper pieces acting as 1mm-thick spacers. The fabric was left in this state for 30 minutes and then removed. Observation of the resulting metal nonwoven fabric using an SEM revealed metal fibers constituting the fabric. Each end of the metal fibers was tapered and curved.

[0106] Example 2 In this example, a copper nonwoven fabric was produced by plating copper onto a copper nonwoven fabric. 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 14.9 g / L. 1850 mL of the solution was placed in an electrolytic cell measuring 12 cm wide x 10 cm deep x 18 cm deep (approximately 2.2 L capacity) and stirred. The electrolyte temperature was 40°C. A 10 cm x 14 cm copper plate was used as the cathode. A 10 cm x 14 cm copper plate was used as the anode. Both electrodes were suspended in the electrolytic cell with a 9.6 cm gap between them. 30 mL of oleic acid was suspended in the electrolyte, and the suspended electrolyte solution was stirred. This allowed the suspended oleic acid to collide with the cathode surface and adhere directly to the cathode surface, thereby exerting its effect. The current density was 313 A / m ２The electrolysis was carried out for 120 minutes with the temperature adjusted to 50°C. In this manner, fibrous copper was electrodeposited on the surface of the cathode. The deposited fibrous copper, which had accumulated on the electrode with little agglomeration, was peeled off and dispersed in 2-propanol to form a slurry. This process was repeated until the required amount of fibrous copper was obtained. Dispersion was carried out using an ultrasonic disperser and the same thin-film rotary high-speed mixer as used in Example 1. A metal nonwoven fabric was produced from the slurry using the same method as in Example 1. Observation of the metal nonwoven fabric using an SEM revealed metal fibers constituting the metal nonwoven fabric. Each end of the metal fibers was tapered and curved. This metal nonwoven fabric was then further copper-plated. A copper pyrophosphate bath was used as the copper plating solution. The copper plating solution contained a potassium pyrophosphate concentration of 349 g / L, a copper pyrophosphate concentration of 82 g / L, and a pH of 8.8 with 0.15 mol / L ammonia water and 0.15 mol / L phosphoric acid. 24 L of copper plating solution was placed in a plating tank measuring 24 cm x 41 cm x 29 cm (volume approximately 29 L) and stirred. The temperature of the plating solution was set to 40°C. A metal nonwoven fabric (20 cm x 30 cm) sandwiched between two 26.5 cm x 31.5 cm copper meshes (20 mesh) was used as the cathode, and copper plating was performed on the meshes. A copper plate measuring 16 cm x 24 cm was used as the anode. Both electrodes were suspended in the plating tank so that the distance between the cathode and anode was 9 cm. The current density was 600 A / m2 relative to the area of ​​the metal nonwoven fabric. ２ The thickness of the nonwoven fabric was adjusted to 1 μm, and electrolysis was performed on both sides for 14.5 minutes. Based on the weight increase of the nonwoven fabric after copper plating, it was shown that copper was plated to a thickness of about 120 nm on the surface of the copper fibers, each of which had a thickness of about 1 μm and constituted the nonwoven fabric before copper plating. When the nonwoven fabric was observed using an SEM, it was found that even after plating, each end of the metal fibers constituting the nonwoven fabric was tapered and curved.

[0107] Example 3 In this example, a metal nonwoven fabric was produced by further plating a copper metal nonwoven fabric with nickel. A metal nonwoven fabric (prior to copper plating) was produced using the same method as in Example 2. This metal nonwoven fabric was then further plated with nickel. A Watts bath was used as the nickel plating solution. A plating solution was prepared with a nickel sulfate concentration of 0.84 mol / L, a nickel chloride concentration of 0.13 mol / L, and a boric acid concentration of 0.49 mol / L. 24 L of this solution was placed in a plating tank measuring 24 cm x 41 cm x 29 cm (approximately 29 L capacity) and stirred. The plating solution temperature was 40°C. A metal nonwoven fabric (20 cm x 30 cm) sandwiched between two 26.5 cm x 31.5 cm copper meshes (20 mesh) was used as the cathode, and the meshes were nickel-plated together. A 16 cm x 24 cm nickel plate was used as the anode. The cathode and anode were suspended in the plating tank so that the distance between them was 9 cm. The current density was 500 A / m ２ The nickel content in the metal nonwoven fabric after nickel plating was approximately 50% by weight. This indicates that nickel was plated to a thickness of approximately 240 nm on the surface of the copper fibers with a thickness of approximately 1 μm that constituted the metal nonwoven fabric before nickel plating. When the obtained metal nonwoven fabric was observed using an SEM, it was found that even after plating, each end of the metal fibers that constituted the metal nonwoven fabric was tapered and curved.

[0108] 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.

[0109] 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.

[0110] Comparative Example 3 An electrolyte solution was prepared from copper sulfate and sulfuric acid so that the copper ion concentration was 4 g / L and the free sulfuric acid concentration was 5 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 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 in an amount of 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 160 A / m ２ The temperature was adjusted to 50°C, and electrolysis was carried out for 30 minutes. In this way, fibrous copper was electrodeposited on the surface of the cathode. The fibrous copper deposited in this way 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. This process was repeated until the required amount of fibrous copper was obtained. 554 mL of the slurry (fibrous copper concentration 100 g / L) was poured into a mold (300 mm length x 200 mm width x 15 mm height) 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 2-propanol was removed from the mold by volatilizing it. The deposit remaining in the mold was dried in a vacuum atmosphere at room temperature (25°C) for 12 hours. A hydrogen-containing atmosphere (3 vol% H ２ Remainder N ２ The deposit was calcined under these conditions without pressure to obtain a metal nonwoven fabric. The calcination was carried out at 350°C for 3 hours. The metal nonwoven fabric produced under these conditions had cracks.

[0111] [Evaluation] For the samples of Examples 1 to 3 and Comparative Example 3, the average fiber diameter, average length, aspect ratio, tip angle, and irregularity rate of the metal fibers constituting the samples were measured using the methods described above. Note that measurements were omitted for the samples of Comparative Examples 1 and 2, which were sponge-like porous bodies (hereinafter also referred to as "sponge metal"). Furthermore, the X / Y values ​​for the metal crystals constituting the metal fibers in the samples of Example 1 and Comparative Example 3 were calculated using the method described above. Note that the X / Y values ​​were not calculated for Comparative Examples 1 and 2. This is because the samples of Comparative Examples 1 and 2 were sponge-like metals, and the X / Y values ​​do not reflect the crystal shape. Furthermore, for the samples of Examples 1 to 3 and Comparative Examples 1 to 3, the compressive modulus, longitudinal length change rate, tensile strength, presence or absence of folding endurance, and standard deviation (SMD) of surface roughness were measured using the methods described above. To measure the longitudinal length change rate, a metal nonwoven fabric measuring 300 mm length x 200 mm width x 1 mm thickness was used, and to measure folding endurance, a metal nonwoven fabric measuring 50 mm length x 10 mm width x 1 mm thickness was used. The standard deviation (SMD) of surface roughness was not measured for the samples of Examples 2 and 3. Furthermore, the longitudinal length change rate of the samples of Examples 1 to 3 was less than 0.1%, and therefore was marked "<0.1" in Table 1. Furthermore, the sample of Comparative Example 3 had insufficient strength, and when a metal nonwoven fabric measuring 300 mm length x 200 mm width x 1 mm thickness was fabricated, many cracks occurred and it could not be molded, so the longitudinal length change rate could not be measured. Therefore, it was marked "unpreparable" in Table 1. In addition, for the samples of Examples 1 to 3 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), and the size of the pores in the metal nonwoven fabric constituting the sample or the size of the pores in the sponge-like metal was determined, and the peak top diameter was obtained. In addition, the porosity was measured using the above-mentioned method. In addition, for the samples of Examples 1 to 3 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. For samples with a specific surface area of ​​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 volume V1 and mass of the sample using the method described above. The compressive modulus was measured using a precision materials testing machine (Instron Model 5848) for Examples 1 to 3 and Comparative Example 1, and a small universal materials testing machine (Instron Model 5565) for Comparative Example 2. The detailed conditions were as described above. The results are shown in Table 1 below. In Table 1, "-" indicates that the measurement was not performed or could not be performed.

[0112]

[0113] According to the present invention, a metal nonwoven fabric having flexibility and dimensional stability can be provided.

Claims

1. A metal nonwoven fabric comprising metal fibers, having a compressive modulus of 4 MPa or less, and when the metal nonwoven fabric is formed into a rectangular shape of 300 mm length x 200 mm width x 1 mm thickness and hung so that the lengthwise direction of the metal nonwoven fabric coincides with the vertical direction, the rate of change in length of the metal nonwoven fabric in the lengthwise direction is 10% or less.

2. A metal nonwoven fabric comprising metal fibers, having a compressive modulus of elasticity of 4 MPa or less, and a tensile strength of 0.01 MPa to 4 MPa or less, wherein the metal nonwoven fabric is formed into a rectangular shape of 50 mm length x 10 mm width x 1 mm thickness, and is placed on a horizontal surface with one end of the metal nonwoven fabric in the longitudinal direction fixed to the horizontal surface, and the other end is pulled up so that the central part of the metal nonwoven fabric in the longitudinal direction has the maximum curvature and the interior angle between the surface of an area at least 10 mm from the other end and the horizontal surface is 120 degrees, without breaking.

3. The metal nonwoven fabric according to claim 1 or 2, wherein the average fiber diameter of the metal fibers is 20 nm or more and 10 μm or less.

4. A nonwoven metal fabric according to claim 1 or 2, in which the void distribution peak top diameter measured by mercury porosimetry is 50 μm or less.

5. A metal nonwoven fabric according to claim 1 or 2, wherein, for the metal crystals constituting the metal fibers, when the length along the direction of extension of the metal fiber is taken as X and the length along the direction perpendicular to said direction is taken as Y, the length of the metal fiber is divided into four equal parts along the direction of extension of the metal fiber, and the arithmetic mean value of X / Y, which is the ratio of X to Y at three points in the boundary region, is 4 or less.

6. A metal nonwoven fabric according to claim 1 or 2, wherein the average length of the metal fibers is 3 μm or more and 5000 μm or less.

7. The metal nonwoven fabric according to claim 1 or 2, wherein the average thickness of the metal nonwoven fabric is 3 μm or more.

8. Specific surface area is 0.02 m ２ / g or more 22m ２ 3. The metal nonwoven fabric according to claim 1, wherein the elastic modulus is 1 / g or less.

9. The metal nonwoven fabric according to claim 1 or 2, 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.

10. A metal nonwoven fabric according to claim 1 or 2, wherein the surface of the metal fibers is coated with a material other than metal.

11. A metal nonwoven fabric according to claim 1 or 2, wherein the metal fibers comprise a core made of a metal and a shell arranged on the surface of the core and made of a metal other than the metal.

12. The metal nonwoven fabric according to claim 1 or 2, wherein the mean deviation SMD of surface roughness measured using a KES-SESRU manufactured by Kato Tech Co., Ltd. in accordance with "Standardization and Analysis of Texture Evaluation," 2nd Edition, by Kawabata Toshio, published by the Texture Measurement and Standardization Research Committee of the Japan Textile Machinery Association on July 10, 1980, is 3 μm or less.

13. An electrode made using the metal nonwoven fabric according to claim 1 or 2.

14. A bonding material comprising the metal nonwoven fabric according to claim 1 or 2.

15. A method for producing a metal nonwoven fabric, comprising: using an electrolyte containing a metal element source, precipitating metal fibers on a cathode by electrolytic reduction in the presence of an oily substance on the surface of the cathode; dispersing the metal fibers and an organic solvent using a thin film spinning method to obtain a slurry; pouring the slurry into a mold to obtain a deposit of the metal fibers; and pressing and firing the resulting metal fiber deposit to obtain a metal nonwoven fabric.

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