Sheet-shaped titanium porous body

Abstract

To provide a sheet-like titanium porous body in which extremely high surface smoothness and low contact resistance are simultaneously realized, and to provide a method for producing the same.SOLUTION: It has a contact resistance of 4m Ω / cm2 or less. At least one of the surfaces has an average pore area of 5 μ m2 or more and 20 μ m2 or less, a standard deviation of pore area of 45 μ m2 or less, and a number of pores of 13.6 or more per 1000 μ m2. The contact resistance may be 1. 0m Ω / cm2 or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] One embodiment of the present invention relates to a sheet-shaped porous titanium body and a method for producing the same. [Background technology]

[0002] Titanium sheet porous bodies containing metallic titanium (zero-valent titanium) are widely used in filters, secondary battery electrodes, polymer electrolyte fuel cell current collectors, catalyst supports, and the like. Titanium sheet porous bodies are produced by sintering titanium powder, but after sintering, an oxide film forms on the surface due to exposure to air and other factors. Therefore, although the metallic titanium that makes up the majority of the titanium sheet porous body has high electrical conductivity, the oxide film present on the surface can increase the electrical resistance (contact resistance) of the area that comes into contact with electrodes and the like. One known method for reducing contact resistance is to fix gold or platinum to the surface of the titanium sheet porous body (see Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-107091 [Patent Document 2] International Publication No. 2020 / 179693 Summary of the Invention [Problem to be solved by the invention]

[0004] An object of one embodiment of the present invention is to provide a porous titanium sheet having a novel structure, or to provide a porous titanium sheet that simultaneously achieves extremely high surface smoothness and low contact resistance. [Means for solving the problem]

[0005] One embodiment of the present invention is a porous titanium sheet. This porous titanium sheet has a resistivity of 1.4 mΩ / cm 2 The titanium porous sheet has a contact resistance of 5 μm or less on at least one surface thereof. 2 More than 20μm 2 The standard deviation of the pore area is 45 μm or less. 2 The number of pores is 1000 μm or less. 2 The average is 13.6 or more. [Brief explanation of the drawings]

[0006] [Figure 1] 1 is a flowchart showing a method for producing a sheet-shaped porous titanium body according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0007] Hereinafter, a sheet-like porous titanium body and a method for producing the same according to an embodiment of the present invention will be described with reference to the drawings. Here, "sheet-like" means a plate-like or foil-like body having a thickness small relative to its dimensions in a plan view.

[0008] 1. Structure and properties of porous titanium sheets 1-1. Structure As will be described in detail later, one embodiment of the present invention involves fabricating a sheet-shaped titanium sintered body from high-purity titanium powder using a paste method, followed by sputtering one or more metals (zero-valent metals) selected from platinum group elements and gold onto at least one surface of the sheet-shaped titanium sintered body. Here, platinum group elements refer to platinum, ruthenium, rhodium, palladium, osmium, and iridium, with platinum being preferred. Therefore, the sheet-shaped titanium porous body includes a high-purity titanium sintered body having numerous pores on its surface and interior, as well as the above-mentioned metals present on the surface of the titanium sintered body and on some of the pore wall surfaces near the surface. For example, because the metal is formed by sputtering, no metal is formed on the interior pore wall surfaces away from the surface, or if any, only a small amount of metal is present. Furthermore, by providing one or more metals selected from platinum group elements and gold on the surface of a titanium sintered body with excellent surface smoothness, the sheet-shaped titanium porous body simultaneously achieves high surface smoothness and low contact resistance.

[0009] The shape, size, and thickness of the titanium porous sheet in plan view can be appropriately determined depending on the application. For example, the titanium porous sheet may be rectangular with sides of 200 mm to 2000 mm in plan view. Alternatively, the titanium porous sheet may be rectangular, such as a square or rectangle, with an aspect ratio of 1:1 to 1:3, or may be rhombic, other polygonal, elliptical, or circular, such as a perfect circle. The corners of a polygonal titanium porous sheet may be chamfered. The thickness may be, for example, 10 μm to 500 μm, 40 μm to 400 μm, or 80 μm to 300 μm. Adjusting the thickness within the above ranges allows the titanium porous sheet to be more suitably used as a porous transport layer in a PEM water electrolysis system, taking advantage of its high surface smoothness and low contact resistance. Known methods may also be used to measure the thickness. For example, the thickness of the porous titanium sheet may be measured at five points, four on the periphery and one in the center, using a thickness gauge with a measurement accuracy of 0.001 to 0.01 mm, and the average value of these measurements may be used as the thickness. If the porous titanium sheet is rectangular in plan view, the four peripheral points may be the four corner points.

[0010] 1-2.Characteristics As described above, the porous titanium sheet is manufactured using high-purity titanium powder as one of its raw materials. Therefore, the titanium sintered body, which accounts for the majority of the porous titanium sheet, exhibits high titanium purity. Furthermore, numerous pores present on the surface and inside the sintered titanium body form numerous interconnected pathways from one side of the porous titanium sheet to the other. In other words, the sintered titanium sheet has a three-dimensional network structure, a sort of titanium sponge-like skeleton, formed by the sintering of titanium powder. This structure provides the porous titanium sheet with high air and liquid permeability, as well as high compression resistance and low brittleness (impact energy absorption) due to the numerous contact points between the powder particles. Furthermore, it is believed that the platinum group elements and / or gold formed on the surface of the sintered titanium body and on some of the inner walls of the pores exhibit extremely low contact resistance. Furthermore, because the paste method is used to fabricate the porous titanium sheet, both or at least one of the surfaces of the porous titanium sheet exhibits extremely high smoothness. On the other hand, titanium sintered bodies made using fibrous titanium powder have a nonwoven fabric-like skeleton, and therefore the ends of the fibers are likely to protrude toward the surface and damage the electrolyte membrane of a PEM water electrolysis device. Because of the above-mentioned properties, the sheet-shaped titanium porous body according to one embodiment of the present invention is useful as a porous transport layer (PTL) in a polymer electrolyte membrane (PEM) water electrolysis device that is exposed to strongly acidic and strongly oxidizing conditions.

[0011] (1) Purity Specifically, the titanium content in the titanium sintered body is, for example, 97% by mass or more and 99% by mass or less. Furthermore, the content of other metal elements (e.g., chromium, aluminum, copper, zinc, tin, iron, etc.) mixed in the raw titanium powder is also low, at 0.3% by mass or less. The oxygen content is also low, at 0.5% by mass or more and 2.0% by mass or less, for example. Furthermore, the metal formed on the surface of the titanium sintered body is a platinum group element or gold, which have low ionization tendency. For these reasons, even when exposed to strongly acidic or strongly oxidizing conditions, the amount of elution of metal elements contained as impurities in the titanium sintered body is extremely low. Therefore, even when the sheet-shaped porous titanium body is used as a porous transport layer in a PEM-type water electrolysis system, it will not contaminate the system. Here, the titanium content in the titanium sintered body can be determined by determining the contents of metal elements other than titanium and oxygen using X-ray fluorescence elemental analysis and inert gas fusion-infrared absorption spectroscopy, respectively, and subtracting the contents of metal elements other than titanium and oxygen from the total amount (100%) of the titanium sintered body.

[0012] (2) Porosity The porosity of the titanium sintered body is, for example, 30% to 60% or 30% to 55%. The porosity is determined by the volume calculated from the width, length, and thickness of the titanium sintered body, the apparent density ρ' of the titanium sintered body calculated from the mass of the titanium sintered body, and the true density ρ of titanium (4.51 g / cm 3 ) and calculated using the formula: ε = (1 - ρ' / ρ) × 100. Because the thickness of the metal formed on the surface of a titanium sintered body is extremely small compared to the size of the titanium powder that is the raw material for the titanium sintered body and the size of the pores (described below), it is reasonable to think that the porosity of the titanium sintered body is maintained and reflected in the titanium porous sheet body. For this reason, the porosity of the titanium porous sheet body is also thought to be within the above-mentioned range.

[0013] (3) Pore The number of pores that the sheet-shaped porous titanium body has on its surface is, for example, 1000 μm 2The number of pores on the surface may be 50 or less. The number of pores is determined, for example, by the number of pores per continuous observation area (for example, an area of ​​10 4 μm 2 Over 10 5 μm 2 Below, typically 2.2 × 10 4 μm 2 The number of pores detected can be obtained by observing the surface of the porous titanium sheet and dividing the number of pores detected by the area of ​​the observation area. The number of pores observed on the surface of the porous titanium sheet is counted as an integer, but this does not necessarily mean that the number per area is an integer. The shape of the observation area can also be any shape, and it can be a polygon including a square or rectangle, a circle or an ellipse, or a shape whose outline is made up of straight lines and curves. For example, if the area of ​​one side of the porous titanium sheet is 2.2 x 10 4 μm 2 In a rectangular observation area with a length:width ratio of 3:4, the number of pores is 300 or more, 350 or more, or 400 or more, and may be 700 or less, or 600 or less. 2 In terms of pores per unit area, the number of pores may be 13.6 or more, 15.9 or more, or 18.2 or more, and may be 31.8 or less, or 27.3 or less.

[0014] The number and size of pores can be calculated by visual observation of the observation area, but automatic detection, including image processing of the observation area, can be used to efficiently calculate the number and size and eliminate measurement variations caused by the operator. Any image processing method can be used as long as it clearly distinguishes pores from other areas. For example, the observation area is binarized. Then, within the area below a certain brightness threshold (i.e., areas with a gradation of 0 in the binary image), the brightness of closed regions of a certain area or less is inverted to convert these regions into white regions (i.e., areas with a gradation of 1 in the binary image). Here, a closed region is a black region surrounded by white regions. On the other hand, within the area above a certain brightness threshold (i.e., white regions), the brightness of closed regions of a certain area or less is inverted to convert these regions into black regions. Here, a closed region is a white region surrounded by black regions. The brightness threshold can be set arbitrarily, for example, to half the maximum brightness in the observation area. Furthermore, the certain area may be appropriately determined based on the area of ​​the observation area, for example, an area of ​​0.003% or less of the observation area. The white areas remaining after this process are considered to be pores, and their number is used as the number of pores. Furthermore, to eliminate in-plane variation in measurements, it is preferable to calculate the number of pores in multiple observation areas that do not overlap each other or that do not overlap at least partially with each other. When the sheet-shaped porous titanium body is rectangular, for example, observation areas can be set at the center and four corners, and the results of these observation areas can be averaged. Automatic detection of the number of pores, including image processing, may be performed by a computing device mounted on or connected to the scanning electron microscope, or by a computing device independent of the scanning electron microscope.

[0015] The porous titanium sheet not only has a large number of pores that are continuous from one surface to the other, but also has small pores that open to the surface. Specifically, the average area of ​​the pores on the surface of the porous titanium sheet, i.e., the average area of ​​the pores that open to the surface, is 5 μm 2 More than 20μm 2 The average pore area is 5 μm 2 More than 18μm 2Below, 10 μm 2 More than 18μm 2 Furthermore, the pore area has a small variation, with a standard deviation of 10 μm or less. 2 More than 45 μm 2 Below, 20μm 2 More than 45 μm 2 Below, 25μm 2 More than 45 μm 2 or less than 30 μm 2 More than 45 μm 2 The minimum standard deviation is, for example, 10 μm as mentioned above. 2 , 20 μm 2 , 25 μm 2 , or 30 μm 2 is.

[0016] Because of the large number of small pores, when a porous titanium sheet is used, for example, in the porous transport layer of a PEM water electrolysis device, the intrusion of adjacent catalyst layers or electrolyte layers into the pores is suppressed, thereby suppressing damage to the catalyst layers or electrolyte layers (i.e., the surface of the porous titanium sheet can be evaluated as smooth). Due to these characteristics, damage can be suppressed, and at the same time, the large number of pores ensures sufficiently high air permeability and liquid permeability. Furthermore, a small standard deviation of the pore area means that there are almost no abnormally large pores, such as pinholes, that deviate significantly from the average. Therefore, the porous titanium sheet can be said to have small in-plane variation in its properties.

[0017] (4) Contact resistance As mentioned above, the titanium sintered body, which accounts for the majority of the titanium sheet-shaped porous body, has high titanium purity. Furthermore, platinum group elements and / or gold, which have high electrical conductivity, are present on the surface of the titanium sintered body and on the wall surfaces of the pores near the surface. Therefore, the contact resistance of the titanium sheet-shaped porous body is low, at 1.4 mΩ / cm. 2 Below, 1.0mΩ / cm 2 Below, 0.7mΩ / cm 2 or less, or 0.5mΩ / cm 2The lower limit of the contact resistance is, for example, 0.1 mΩ / cm 2 The above is the entire explanation. Here, contact resistance refers to the electrical resistance when a conductor is in contact with a titanium sheet-like porous body and a current is applied. Contact resistance can be measured by sandwiching a titanium sheet-like porous body (sample size, for example, 10 mm square in plan view) between a pair of metal plates (e.g., copper plates) and applying a pressure (e.g., 200 N to 500 N, more specifically, 450 N) in the thickness direction while applying a current (e.g., 1 A direct current) between the pair of metal plates. Preferably, for more accurate measurements, contact resistance is measured using the four-terminal method. The contact resistance is the resistance obtained from the current value flowing between the pair of metal plates when the current value is stable (usually stable 1 to 10 minutes after the start of current application) and the voltage between the pair of metal plates. The measured value also includes the resistance in the thickness direction, but because the surface resistance is dominant, the resistance in the thickness direction can be ignored for thicknesses of approximately 1000 μm or less. In addition, in the above measurement method, in order to eliminate in-plane variation in the measurement, a sheet of porous titanium may be divided, and the contact resistance may be measured at multiple points on each divided sample (for example, at the center and four corners, for a total of five points), and the average of these measurements may be used as the contact resistance of the porous titanium sheet.

[0018] (5) Smoothness As described above, the sheet-shaped porous titanium body has a large number of small pores. The surface of the titanium sintered body, particularly the surface that was in contact with the substrate (described later) has extremely high smoothness. Because of this extremely high smoothness, when the sheet-shaped porous titanium body is used as a porous transport layer in a PEM water electrolysis system, for example, it can prevent damage to components adjacent to or located near the porous transport layer, such as a catalyst layer or electrolyte layer, thereby contributing to maintaining the characteristics of the water electrolysis system over a long period of time and improving the life of the system.

[0019] 2. Manufacturing method of porous titanium sheet The titanium sheet porous body is produced by forming a titanium sintered body using a paste method, followed by forming a metal on the surface of the titanium sintered body and the walls of its pores. Specifically, as shown in the flowchart in Figure 1, a titanium paste is first prepared containing fine titanium powder with a particle size of approximately 10 μm to 20 μm, polyvinyl butyral as a binder, a monohydric alcohol as an organic solvent, and a dispersant containing coal tar naphtha (the dispersant content in the paste is, for example, 0.01% by mass or more and 0.3% by mass or less), but containing no water or a blowing agent. The titanium paste is then applied to a substrate in sheet form, and the solvent contained in the titanium paste is evaporated (i.e., the titanium paste is dried at a temperature of approximately 110°C to 150°C), resulting in a green body formed on the substrate. The green body is then peeled off from the substrate and heated to decompose (degrease) at least a portion of the binder and additives contained in the titanium paste, resulting in a brown body. The brown body is sintered at a temperature of approximately 775°C to 850°C, resulting in a sheet-shaped titanium sintered body essentially consisting of titanium. The sheet-shaped titanium sintered body produced in this way has excellent surface smoothness. By forming one or more metals selected from platinum group elements and gold on one or both surfaces of the titanium sintered body by sputtering, a sheet-shaped titanium porous body can be produced. These steps are described in detail below.

[0020] 2-1.Preparation of titanium paste Titanium paste is a fluid containing the fine titanium powder described above, and in addition to the titanium powder, also contains a binder, an organic solvent, and a dispersant. Titanium paste may contain one or more additives such as a plasticizer and a leveling agent. Note that titanium paste does not contain a foaming agent because foaming during the manufacturing process would cause the surface smoothness of the titanium sintered body to be lost. For the same reason, titanium paste does not contain water, although unavoidable water contamination during the manufacturing process of titanium paste is acceptable.

[0021] (1) Titanium powder As mentioned above, the particle size of the titanium powder is about 10 μm to 20 μm. That is, it is preferable to use titanium powder with a particle size of 10 μm or more and 20 μm or less, obtained by sieving or air classification. By using such fine titanium powder, a titanium sintered body with excellent smoothness and air or liquid permeability can be obtained.

[0022] The titanium powder is preferably hydrogenated / dehydrogenated titanium powder (also known as HDH powder), which is obtained by hydrogenating and crushing titanium sponge, titanium scrap, titanium castings, and forgings, followed by dehydrogenation. However, because titanium powder is fine, its oxygen content alone tends to be higher than that of commercially pure titanium. For example, the titanium content of titanium powder is 97% by mass or more, 98% by mass or more, or 99% by mass or more but 100% by mass or less, and the oxygen content is 0.1% by mass or more but 0.7% by mass or less, or 0.6% by mass or less. Using titanium powder that is fine yet has a low oxygen content can produce a titanium sintered body with high electrical conductivity and further contribute to the extremely low contact resistance of the sheet-shaped titanium porous body. In addition to oxygen, the titanium powder may contain trace amounts of other components, such as nonmetallic elements such as carbon and nitrogen, or metallic elements such as chromium, aluminum, copper, zinc, tin, and iron.

[0023] (2) Binder It is preferable to use polyvinyl butyral as the binder. Furthermore, various polymeric materials can be used in addition to polyvinyl butyral. Examples include polymers having a basic skeleton such as alkylated celluloses, such as methyl cellulose and ethyl cellulose, acrylic polymers, such as poly(meth)acrylic acid esters, and polyvinyl alcohol. In preparing the titanium paste, one type of binder may be used, or two or more types of binders may be used.

[0024] (3) Organic solvent As the organic solvent, a monohydric alcohol having about 1 to 5 carbon atoms is typically used. Examples include alcohols having 5 or less carbon atoms, such as ethanol, n-propanol, isopropyl alcohol, n-butanol, isobutyl alcohol, and sec-butanol. In addition, monoterpene alcohols such as terpineol, ethylene glycol monoalkyl ethers such as butyl carbitol, aromatic hydrocarbons such as toluene, xylene, trimethylbenzene, and tetralin, linear, branched, or cyclic aliphatic hydrocarbons such as hexane, heptane, octane, and cyclohexane, ketones such as methyl ethyl ketone and cyclohexanone, and amide solvents such as N-methylpyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide can also be used.

[0025] (4) Dispersant The titanium paste contains a dispersant. This dispersant contains coal tar naphtha. The coal tar naphtha in the dispersant is believed to adhere to the surface of the titanium powder, making it easier for the titanium powder particles to slide against each other and improving the dispersion of the titanium powder in the titanium paste. This suppresses aggregation or localization of the titanium powder in the titanium paste, allowing small-particle titanium powder to be dispersed and bonded relatively uniformly during sintering. As a result, a titanium sintered body is obtained with numerous fine pores formed on at least one surface. Furthermore, coal tar naphtha is a hydrocarbon mixture with a relatively low boiling point and is believed to volatilize along with the organic solvent and binder during drying and degreasing, so it does not significantly affect the titanium sintered body.

[0026] 2-2. Application and drying Next, the titanium paste is applied to a substrate in the form of a sheet. The thickness of the applied titanium paste can be adjusted taking into account the thickness of the titanium sintered body or the titanium porous sheet. Examples of substrates include polyesters such as polyethylene terephthalate and polyethylene naphthalate, polyolefins such as polyethylene, polypropylene, and polystyrene, polyvinyl acetate-based polymers such as polyvinyl acetate and partially saponified polyvinyl acetate, and polylactic acid. Among these, polyethylene terephthalate is preferred because it can be molded into a film with high flexibility and sufficient strength, and is inexpensively available.

[0027] The titanium paste application method is not limited, and various techniques, such as doctor blade, lip coating, offset printing, and gravure printing, may be used. For example, various film-forming devices, such as a rear doctor coater, blade coater, rod coater, knife coater, squeeze coater, impregnation coater, reverse roll coater, transfer roll coater, gravure coater, kiss roll coater, slot die coater, cast coater, spray coater, curtain coater, calendar coater, extrusion coater, and bar coater, may be used. Among these, the doctor blade method, which allows continuous application of titanium paste to a long substrate, is preferred. For long substrates, a sheet-like green body can be wound into a roll, which is advantageous for storage and transportation. A release agent may be applied to the substrate before applying the titanium paste. Alternatively, a substrate pre-applied with a release agent may be used. Examples of release agents include a solution or dispersion of a fluororesin or silicone resin. Applying a release agent facilitates separation of the dried titanium paste from the substrate.

[0028] The applied titanium paste is then dried at a temperature of approximately 110°C or higher and 150°C or lower. This means that the organic solvent contained in the titanium paste is volatilized. This results in a green body being obtained on the substrate. Drying can be carried out under either normal pressure or reduced pressure. Since the organic solvent volatilizes from the titanium paste, it is preferable to quickly remove the volatilized organic solvent by blowing air over the titanium paste on the substrate. The drying atmosphere can be set as desired, and drying can be carried out in an atmosphere of air, nitrogen, oxygen, helium, argon, or a mixture of these gases.

[0029] 2-3. Degreasing Next, debinding is performed to remove binders and additives from the green body. Specifically, the green body is peeled off from the substrate and heated (preheated). There are no restrictions on the method for peeling the green body; it can be physically peeled off from the substrate. For example, one end of the substrate and green body stack can be bent, adhesive tape or a roller with an adhesive surface can be used to peel off the substrate or green body, compressed air can be blown between the substrate and green body, or a sharp knife-like tool can be inserted to peel off a portion of the substrate from the green body, and the peeled portion can be used as a starting point to separate the green body from the substrate. Heating can be performed, for example, on a graphite plate called a setter.

[0030] Debinding is performed at a temperature at which the binder and additives decompose and / or volatilize. For example, debinding can be performed by heating the green body in an oxygen-containing atmosphere, such as air, at a temperature between 300°C and 450°C for a period of 2 to 20 hours. This debinding process can remove all or most of the binder and additives from the green body. The sheet-like compact obtained by debinding the green body is called a Brownian body.

[0031] 2-4.Sintering A titanium sintered body is obtained by sintering the debound brown body. Sintering is performed at a higher temperature than debinding, and the sintering temperature is appropriately selected from a range of temperatures between 775°C and 850°C. Sintering is performed at atmospheric pressure or reduced pressure. When sintering at atmospheric pressure, it is preferable to sinter the brown body in an atmosphere with low concentrations of oxygen and nitrogen, for example, a rare gas atmosphere such as helium or argon, in order to prevent the formation of oxides and nitrides. When sintering under reduced pressure, the pressure is, for example, 1×10 -4 Pa or more 1×10 -2 The sintering time may also be set appropriately depending on the thickness of the brown body, and may be selected, for example, from the range of 1 hour to 8 hours.

[0032] 2-5. Acid treatment As shown in Figure 1, an optional acid treatment may be performed before the metal formation. This is because an oxide coating forms on the surface and pore walls of the titanium sintered body obtained by sintering. By removing at least a portion of this coating before forming the metal, the increase in contact resistance due to the coating can be prevented or reduced. The acid treatment can be performed by treating the titanium sintered body with an acid containing hydrofluoric acid, which can dissolve titanium oxide. Specifically, the acid treatment is performed by immersing the titanium sintered body in hydrofluoric acid or a mixture of hydrofluoric acid and an inorganic acid such as nitric acid, sulfuric acid, yellow water, or hydrobromic acid. For example, the acid treatment can be performed by immersing the titanium sintered body in a solution called Kroll's solution, which contains nitric acid, hydrofluoric acid, and water, at room temperature for 0.5 to 10 minutes. After the acid treatment, the titanium sintered body is washed with water. After washing with water, the porous titanium body may be further washed with a low-boiling point solvent that is compatible with water, such as methanol, ethanol, or acetone, to remove the water. The acid treatment removes at least a portion of the oxide coating. Furthermore, because metallic titanium dissolves in hydrofluoric acid, some of the metallic titanium may also dissolve depending on the conditions of the acid treatment. As a result, although the pore size increases, the air and liquid permeability can be improved while maintaining high surface smoothness.

[0033] 2-6. Metal formation The metal can be formed, for example, by sputtering. Examples of sputtering methods include DC sputtering, radio frequency sputtering, and magnetron sputtering, and any of these methods can be used. Specifically, a titanium sintered body obtained by sintering a Brownian body (or a titanium sintered body further treated with acid) and a sputtering target containing a metal selected from platinum group elements and gold are placed in the chamber of a sputtering apparatus, and the sputtering target is sputtered under reduced pressure to form the metal. The sputtering target may contain a single metal, or a sputtering target containing multiple metals (i.e., an alloy). Multiple sputtering targets with different compositions may also be used. In this case, sputtering may be performed simultaneously on sputtering targets with different compositions, or multiple sputtering targets with different compositions may be sputtered sequentially. In the latter case, the metal formed on the titanium sintered body is composed of multiple layers with different compositions.

[0034] The thickness of the metal formed can usually be controlled by the sputtering conditions. The metal may be formed on only one side of the titanium sintered body, or on both sides. In the former case, it is preferable to form the metal on the side where the green body, which has higher smoothness, was in contact with the substrate. In sputtering, metal atoms ejected from the target are deposited on the surface of the titanium sintered body. Furthermore, the pore structure of titanium sintered bodies is complex. Therefore, it is thought that the metal atoms do not reach the pores present inside the titanium sintered body, but are formed on the surface of the titanium sintered body and on the wall surfaces of the pores nearby.

[0035] The above series of steps allows the production of a porous titanium sheet according to one embodiment of the present invention. As described above, the porous titanium sheet has numerous small pores that connect from one surface to the other, and the pore sizes vary little. Therefore, the surface of the porous titanium sheet, particularly the surface where the green body was in contact with the substrate, has extremely high smoothness, and the porous titanium sheet exhibits excellent liquid permeability and air permeability. Furthermore, one or more metals selected from platinum group elements and gold are provided on the surface of the porous titanium sheet and on a portion of the pore walls. Therefore, the contact resistance of the porous titanium sheet is extremely low, and can be reduced to, for example, approximately 10% of the contact resistance of a sintered titanium body. Thus, the porous titanium sheet according to one embodiment of the present invention simultaneously achieves high surface smoothness and low contact resistance, making it useful as a functional material such as a secondary battery electrode, a polymer electrolyte fuel cell current collector, or a catalyst support, particularly as a porous transport layer in a PEM water electrolysis device. [Example]

[0036] In this example, a porous titanium sheet was produced and its properties were evaluated.

[0037] 1. Preparation of porous titanium sheet A titanium paste containing HDH titanium powder with a particle size of 10 μm or more and 20 μm or less, polyvinyl butyral as a binder, isopropyl alcohol as an organic solvent, and SN Sparse 2190 (made by San Nopco Ltd.) as a dispersant (containing 42 mass% coal tar naphtha and 58 mass% polycarboxylic acid amine salts, etc.) was prepared, coated into a sheet, and dried at 110°C to obtain a green body. This green body was degreased and sintered at 800°C to obtain a sheet-like titanium sintered body (thickness 250 μm).

[0038] Platinum and palladium were deposited on both sides of the prepared titanium sintered body samples (250 μm thick, 10 mm × 10 mm) by sputtering. Specifically, a JEOL sputtering system (JUC-5000 magnetron sputtering) was used, and sputtering was performed using a target containing platinum and palladium (Pt:Pd = 80:20 w / w) at a discharge current of approximately 10 mA and a pressure of approximately 6 Pa. The samples obtained by sputtering each side of the titanium sintered body for 90 and 180 seconds are referred to as Examples 1 and 2, respectively. The titanium sintered body samples were then treated at room temperature with Kroll's solution (6% nitric acid, 3% hydrofluoric acid, 91% water, by volume), followed by deposition of platinum and palladium on both sides of the titanium sintered body in the same manner as in Example 1. The samples obtained by acid treatment for 60, 120, and 180 seconds are referred to as Examples 3, 4, and 5, respectively. A sample that was not subjected to either acid treatment or metal formation, i.e., the above titanium sintered body, was used as a comparative example.

[0039] 2. Characterization of porous titanium (1) Measurement of pore area In each of Examples 1 to 5 and the Comparative Example, an electron microscope image (magnification 2000 times) of the surface that had been in contact with the substrate was taken using a scanning electron microscope (Keyence Corporation, ultra-deep multi-angle lens VHX-D510). 2The observation area, with a vertical:horizontal ratio of 3:4, was subjected to binarization. The threshold value was set at half the maximum brightness in each observation area. For closed regions that became black through the binarization process (i.e., regions with a brightness less than half the maximum brightness in the observation area and a gradation of 0 in the binary image) and whose area was less than 0.003% of the observation area, the brightness was inverted and converted to white regions (i.e., regions with a gradation of 1 in the binary image). On the other hand, for closed regions that became white through the binarization process (i.e., regions with a brightness greater than half the maximum brightness in the observation area and a gradation of 1 after binarization) and whose area was less than 0.003% of the observation area, the brightness was inverted and converted to black. The white regions obtained through this process were considered to be pores, and their number, area, and the average and standard deviation of the areas were calculated. These processes were performed using software installed in the electron microscope image. In addition, when measuring each sample, observation areas were set in the center and four corners of each sample, and the average value of the results obtained in these five observation areas was used as the measurement result for the pores in each sample.

[0040] (2) Contact resistance measurement After cleaning the sample with acetone, it was sandwiched between a pair of copper plates (approximately 5 mm thick) and a pressure of 450 N was applied in the thickness direction. In this state, a digital resistance meter (Tsuruga Electric Co., Ltd. Digital Resistance Meter 356G-X-03-H) was connected between the pair of copper plates, and the resistance between the copper plates was measured using the DC four-terminal method. The resistance value was taken when the measured value stabilized (approximately 5 minutes after the start of measurement). The contact resistance was calculated by dividing the obtained resistance by the area of ​​the sample.

[0041] (3) Results and Discussion The measurement results are shown in Table 1. As shown in Table 1, it was confirmed that all of the Examples exhibited low contact resistance compared to the contact resistance of the Comparative Example. For example, when Examples 4 and 5 were compared with the Comparative Example, the contact resistance was reduced by approximately 90%, to 0.2 mΩ / cm 2It was found that the contact resistance was reduced to the order of 1 / 2. It is also estimated that the contact resistance decreases as the sputtering time increases, i.e., as the metal thickness increases (Examples 1 and 2). A comparison of Examples 1 and 3 confirmed that acid treatment reduces contact resistance even more effectively. This is thought to be because the acid treatment removes part of the oxide film, increasing the contact area between the titanium metal and the metal. A comparison of Examples 2 to 4 also shows that the longer the acid treatment time, the more the contact resistance decreases. However, a comparison of Examples 4 and 5 suggests that the contact resistance does not change significantly if the acid treatment is performed for a certain period of time. Under the conditions used in this example, it is thought that a 120-second acid treatment was able to most effectively reduce the oxide film present on and near the surface of the titanium sintered body.

[0042] The results in Table 1 suggest that the sputtering time does not significantly affect the pore area. This indicates that the pores are not filled by sputtering, and the surface smoothness and air / liquid permeability of the titanium sintered body are maintained even after metal formation. This suggests that the size and porosity of the pores on the surface of the sheet-shaped porous titanium body do not substantially change after sputtering. The results of Examples 3, 4, and 5 indicate that the pore area increases with increasing acid treatment time, and the standard deviation also increases. This is thought to be because the acid treatment removes the oxide film and dissolves part of the metallic titanium, resulting in an increase in pore size. In other words, the pore size can be controlled by appropriately selecting the acid treatment conditions. Therefore, when aiming to achieve both high surface smoothness and low contact resistance, it is useful to first prepare a titanium sintered body with excellent surface smoothness and then reduce the contact resistance.

[0043] [Table 1]

[0044] The above results clearly demonstrate that by applying the embodiments of the present invention, it is possible to produce a sheet-shaped porous titanium body having a large number of small pores and low contact resistance. Furthermore, as described above, the sheet-shaped porous titanium body according to the embodiments of the present invention is produced using a paste method, and therefore has a high level of surface smoothness. Therefore, by applying the embodiments of the present invention, it is possible to provide a sheet-shaped porous titanium body that combines low contact resistance and high surface smoothness.

[0045] Based on the above-described embodiments of the present invention, those skilled in the art may add, delete, or modify components, or add, omit, or modify processes as appropriate, as long as they comply with the spirit of the present invention. Even if there are other effects and advantages different from those achieved by the aspects of the above-described embodiments, those that are clear from the description in this specification or that can be easily predicted by those skilled in the art are naturally considered to be achieved by the present invention.

Claims

【Request Item 1】 Contact resistance is 1.4 mΩ / cm ２ is as follows: On at least one surface, the average area of ​​the pores is 5 μm ２ 20 μm or more ２ The standard deviation of the area is 45 μm or less. ２ The number of pores is 1000 μm or less. ２ The porous titanium sheet has a surface roughness of 13.6 or more.

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