A METHOD FOR PREPARING METAL FIBER ARTICLES

Abstract: A method for preparing metal fiber articles is disclosed. The method comprises forming a metal coating on an organic fiber fabric by plating and firing the thus coated fabric in a reducing gas atmosphere. Also a method for preparing a cadmium electrode with a nickel substrate utilizing said method is disclosed.

Description: Field of the Invention This invention relates to a method for preparing flexible metal fiber articles used as electrodes, various filters, parabola antennas, and the like. Background of the Invention As processes for manufacturing metal fibers, there have been known a method comprising repeatedly drawing metal threads as in the case of production of copper fibers, vibration cutting (lathing) of a metal rod as in the case of the production of nickel fibers, a method comprising extruding a viscoelastic composition comprising powder of a reducible metal compound, a binder, a dispersant, etc. in the form of fibers, and thereafter firing the extruded material in a reducing atmosphere, etc. Metal fiber articles are manufactured by forming the thus produced metal fibers into a fabric (woven, nonwoven or knit fabric ) as described in JP-A56-35702, etc. However, all these methods comprise a plurality of complicated process steps, wherein the metal fibers are easily fatigued, and, therefore, working or fabricating thereof is difficult and uniform products cannot be regularly produced. Also it is known to manufacture foamed metal materials such as foamed nickel by plating a foamed resin body with nickel for instance, firing the plated foamed resin in the atmosphere and reducing the formed nickel oxide. This method is also complicated in the process steps and the products obtained is too hard to be used easily although the products are uniform. These metal fiber articles are utilized because of their electric conductivity and high porosity (void fraction). However, the porosity is 95% at the highest in the product of JP-A56-35702, which is made by forming a nonwoven fabric of nickel fibers and sintering it with nickel powder. That of the nonwoven fabric of the nickel fibers made by vibration cutting is 91% and that of the foamed nickel is 93%. The metal fiber articles other than the foamed nickel exhibit obvious non-uniformity in the thickness of fibers and in the porosity. From the FR-A-1 510 317 there is known a process for producing metal fiber articles whereby an organic fiber is impregnated with a metal compound solution. Then the impregnated fiber is slowly heated in an oxygen-containing atmosphere to decompose the fiber and to get rid of most part of carbon to build up a tungsten oxide skeleton, followed by heating further in a reducing atmosphere to temperatures sufficient to obtain the metal, i.e. tungsten, in metallic state. This invention is intended to solve the above-described problems, that is, the complexity of manufacturing process steps, non-uniformity in the product, difficulty of making articles having porosity of higher than 95%, lack of flexibility in the final products (hardness and brittleness) and to provide an improved process for a cadmium electrode with a nickel substrate. Under the circumstances, we intensively studied in search of measure for solving the problems and have found that the production process steps are simplified, uniform and flexible products having void rate up to about 98% can be obtained by firing in a reducing gas atmosphere organic fiber fabric (woven, nonwoven and knit) plated with a metal. As a result of the study, we have found that an excellent cadmium electrode with nickel substrate can be easily obtained by said process. Summary of the Invention This invention provides a method for preparing flexible metal fiber articles comprising forming a metal coating on a piece of an organic fiber fabric by plating and heating the thus coated fabric in a reducing gas atmosphere. Also, this invention provides a process for preparing a cadmium electrode with a nickel substrate. Therefore, the object of the invention is a method for preparing flexible metal fiber articles comprising forming metal on the fibers of an organic fiber fabric and heating the fabric in a reducing gas atmosphere so as to remove the organic fibers, wherein the fibers are plated with the desired metal by (i) electroless plating, (ii) electroless plating followed by electrolytic plating, (iii) vapor deposition plating followed by electroless plating, or (iv) vapor deposition plating followed by electrolytic plating; and heating the thus plated fabric in a reducing gas atmosphere at 250-1200°C. This process of the present invention is applicable to all metals that can be precipitated from an aqueous solution. Preferred are Cu, Ni, Ag and Co. In the process of the present invention, the term "fabric" encompasses woven, nonwoven and knit fabrics. As the method of plating, any of electroless plating, electrolytic plating and vapor deposition plating and any combination thereof can be employed. The plating step is started with the refining using a surfactant. When only electroless plating is employed, the refined fabric is activated with a Sn/Pd catalyst system, whereafter it is immersed in an electroless plating solution containing a metal salt, a complexing agent, a reducing agent, etc. As a reducing agent, sodium borohydride, dimethylamine, borane, sodium hypophosphite, hydrazine and derivatives thereof, formalin, etc. can be used. In order to obtain pure metal and from the viewpoint of ease of handling, formalin is preferred in the case of Cu, and hydrazine and derivatives thereof are preferred in the case of Ni and Co. Usually electrolytic plating is applied after the scouring and vapor deposition or electroless plating. However, sometimes electroless plating is employed after vapor deposition plating. Commercially available scouring agents, pretreatment agents, electroless plating solutions, brightening agents, additives, etc. can be used. Organic fibers consisting of elements C and H; C, H and O; or C, H, O and N are preferred. Especially preferred are organic fibers consisting of the elements C, C and H, or the elements C, H and O. Organic fibers containing other elements are apt to char or leave ash. Typical fibers constituted of C and H are fibers of polyolefins such as polyethylene and polypropylene. Those of C, H and O are rayon fibers, acetate fibers, polyvinyl alcohol fibers, polyester fibers, etc., and those of C, H, O and N are polyamide fibers and acrylonitrile fibers. Preferred are polyethylene, polypropylene and acrylic fibers. As a reducing gas, hydrogen gas, ammonia gas, carbon monoxide gas, thermally cracked ammonia gas or any mixture thereof can be used. The flow rate of the reducing gas must be varied in accordance with the size of the furnace used, heating temperature, etc. and cannot be simply specified. There is a tendency for the heating rate to increase as the gas flow rate increases. For the sake of regulation of reaction rate the reducing as can be diluted with an inert gas such as nitrogen, argon, etc. Heating is conducted at 250°C - 1200°C, preferably 300°C - 1200°C, and more preferably 500°C - 1000°C. At temperatures below 250°C, too long time is required, and yet heating is often incomplete. At temperatures over 1200°C, the material is excessively sintered and loses flexibility. The heating time depends upon the temperature and, therefore, it cannot be specifically defined, but is generally between 15 minutes and 4 hours, preferably 30 minutes and 2 hours. The electroless plating can be carried out by any known conventional process. For instance, a substrate fiber fabric is scoured, activated with a SnCl₂/PdCl₂ catalyst, and thereafter is immersed in an electroless plating solution containing a Ni salt, a complexing agent, a reducing agent, a pH-adjuster, a stabilizer, additives, etc. The thus produced metal fiber articles can be further plated with another metal or metals by processes known per se. Needless to say, the formed flexible metal fiber articles cannot be exposed to a temperature at which the plated metal or metals deteriorate. When a cadmium electrode with a nickel substrate is prepared, electrolytic plating of Cd can be effected by an ordinary process since Ni has good electric conductivity. That is, Ni coating of the fiber substrate is effected by connecting it to the cathode and immersing it in a plating solution containing cadmium oxide, sodium cyanide, any additives, etc. and applying electric current to the plating bath. The reducing gas is practically ammonia gas, hydrogen gas, or a mixture thereof or one of these diluted with an inert gas such as argon. The heating is carried out at a temperature between 250°C - 300°C. At temperatures lower than 250°C, excessively long time is required and often heating is incomplete. At temperatures over 300°C, Cd is apt to volatilize. The heating time is 1 - 10 hours, preferably 2 - 5 hours. Preferred Embodiments of the Invention The invention will be explained specifically by way of preferred working examples. Example 1 A piece of nonwoven fabric (50x50x3mm) made of 3d rayon fibers and weighing 1.42g was scoured and immersed in 500ml of a solution containing 10g/ℓ SnCl₂ and 10mℓ/ℓ HCl for 10 minutes. The thus treated fabric was activated by immersion in 500mℓ of an aqueous solution containing 1g/ℓ PdCl₂ and 1mℓ/ℓ HCl for 10 minutes, thereafter it was immersed in 500mℓ of an electroless nickel plating solution of the following composition at 80°C for deposition of Ni. Nickel sulfate 18g/ℓ Sodium citrate 10g/ℓ Hydrated hydrazine 50mℓ/ℓ Ammoniacal water (25%) 100mℓ/ℓ After the treated fabric was rinsed with water and dried, it weighed 3.38g, which proved that 1.96g of Ni was deposited. The plated fabric was placed in a furnace and heated at 800°C for 1 hour as hydrogen gas was passed through the furnace at the rate of 2ℓ/min After cooling, it weighed 1.93g and consisted of Ni only in the state of flexible nonwoven fabric. The size was 41x43x2.8mm and the porosity was 97.7%. Example 2 A piece of nonwoven fabric (50x50x3mm) made of 1.5d polyethylene fibers and weighing 1.27g was scoured in the same manner as in Example 1, activated with a Sn/Pd catalyst system. This fabric was immersed in 500mℓ of an electroless copper-plating solution of the following composition at 30°C for deposition of copper. Copper sulfate 12g/ℓ Rochelle salt 50g/ℓ Sodium hydroxide 30g/ℓ Formalin 50ml/ℓ After rinsing and drying, the treated fabric weighed 2.76g, which means that 1.49g of copper deposited. The plated fabric was placed in a furnace and heated at 500°C for 2 hours as NH₃ gas was passed at the rate of 2ℓ/min. After cooling, it weighed 1.48g and consisted of copper only in the state of a flexible nonwoven fabric. The size was 37x39x2.7mm and the porosity was 95.7%. Example 3 A piece of woven fabric (50x50x1.5mm) made of 2d polypropylene fibers and weighing 1.32g was scoured and activated with a Sn/Pd catalyst system in the same manner as in Example 1. The thus treated fabric was immersed in 200ml of an electroless cobalt-plating solution of the following composition which was warmed to 80°C for deposition of cobalt. Cobalt chloride 23.5g/ℓ Rochelle salt 100g/ℓ Hydrated hydrazine 50mℓ/ℓ Sodium hydroxide 50g/ℓ After rinsing and drying, it weighed 2.17g, which means that 0.85g of cobalt deposited. The thus plated fabric was placed in a furnace, and heated at 700°C for an hour as a gas obtained by thermal cracking of ammonia gas was passed through the furnace at the rate of 2ℓ/min. After cooling, it weighed 0.85g and consisted of cobalt only in the state of a flexible woven fabric. The size was 46x47x1.4mm and the porosity was 96.8%. Example 4 A piece of nonwoven fabric (50x50x2mm) made of 1.5d rayon fibers and weighing 1.87g was scoured, activated with a Sn/Pd catalyst system in the same manner as in Example 1. The thus treated fabric was immersed in 250ml of an elctcroless silver-plating solution of the following composition at 25°C for deposition of silver. Silver nitrate 7.9g/ℓ EDTA disodium salt 50g/ℓ Ammonia water 50mℓ/ℓ Formalin 50g/ℓ After rinsing and drying, the treated fabric weighed 3.10g, which means that 1.23g of silver deposited. The thus plated fabric was placed in a furnace and heated at 600°C for 2 hours as hydrogen gas passed through the furnace at the rate of 2ℓ/min. After cooling, it weighed 1.23g and consisted of silver only in the state of a flexible nonwoven fabric. The size was 43x44x1.8mm and the porosity was 96.6%. The porosity, electric resistivity, uniformity and flexibility of the products of the above Examples 1 - 4 and those of foamed nickel, nonwoven fabric of nickel fiber which was prepared by vibration lathing and nonwoven fabric of nickel fiber which was chemically prepared by the process disclosed in JP-A56-35702(1981) are shown in the following table for comparison. Example 5 A 25cm² (5x5cm) piece of nonwoven fabric made of 3d rayon fibers and weighing 1.51g was scoured and immersed in 500ml of an aqueous solution containing 10g/ℓ SnCl₂ and 10mℓ /ℓ HCl for 10 minutes. The thus treated fabric was activated by immersing in 500mℓ of an aqueous solution containing 1g/ℓ PdCl₂ and 1mℓ/ℓ HCl for 10 minutes, whereafter it was immersed in 500mℓ of an electroless plating solution of the following composition at 80°C for deposition of Ni. Nickel sulfate 18g/ℓ Sodium citrate 10g/ℓ Hydrated hydrazine 50mℓ/ℓ Lead acetate 1mg/ℓ Ammonia water 100mℓ/ℓ After the fabric had been kept in the solution for reaction until the Ni in the solution was consumed, it was taken out and washed with water and applied on a stainless steel plate. The stainless steel plate bearing the treated fabric was immersed in a solution of the following composition Cadmium oxide 25g/ℓ Sodium cyanide 120g/ℓ Dextrin 1g/ℓ and subjected to electrolysis with 5A/dm² electric current density at room temperature for 20 minutes. The resulting product was rinsed with water and dried. It weighed 5.29g. The product was heated in a furnace through which ammonia gas was passed at the rate of 3ℓ/min at 290°C for 2 hours. After cooling, a metal fabric in the exact same shape as the original nonwoven fabric weighing 3.76g remained. Chemical analysis revealed that it consisted of 1.97g Ni and 1.79g Cd. Example 6 A 25cm² (5x5cm) piece of nonwoven fabric made of 1.5d acryl fibers and weighing 1.33g was scoured and activated with a SnCl₂/PdCl₂ catalyst system in the same manner as in Example 5, and was further plated with nickel and cadmium under the same conditions. The resulting product weighed 5.17g after washing with water and drying. This was heated in a furnace through which hydrogen gas was flown at the rate of 3ℓ/min at 290°C for 5 hours. After cooling, a metal fabric in the exact same shape as the original nonwoven fabric weighing 3.81g remained. Chemical analysis revealed that it consists of 1.92g Ni and 1.89g Cd.

Claims: A method for preparing flexible metal fiber articles comprising forming metal on the fibers of an organic fiber fabric and heating the fabric in a reducing gas atmosphere so as to remove the organic fibers, wherein the fibers are plated with the desired metal by (i) electroless plating, (ii) electroless plating followed by electrolytic plating, (iii) vapor deposition plating followed by electroless plating, or (iv) vapor deposition plating followed by electrolytic plating; and heating the thus plated fabric in a reducing gas atmosphere at 250-1200°C. The method according to claim 1, wherein at least one of Cu, Ni, Ag, and Co is plated. The method according to one of the claims 1 or 2, wherein the organic fiber consists of the elements C and H; the elements C, H and O or the elements C, H, O and N. The method according to claim 3, wherein the organic fiber consists of the elements C and H or the elements C, H and O. The method according to claim 4, wherein the organic fiber is selected from polyethylene, polypropylene and acrylic fiber. A method for preparing a Cd-electrode with Ni-substrate comprising forming a Ni coating on a piece of an organic fiber fabric, wherein the coating is performed by electroless plating, forming a Cd coating on the thus formed Ni coating by electrolytic plating and heating the thus treated fabric in a reducing gas atmosphere at 250 to 300°C.

Inventors: HIROOKA, MASAKI, NAKABAYASHI, AKIRA, SHIBUTA, DAISUKE, YAMASAKI, HIROAKI, YOSHIZUMI, MOTOHIKO
Applicants: JAPAN VILENE COMPANY,LTD, MITSUBISHI MATERIALS CORPORATION
Application year: 1988
Main classification: C23C18
IPC codes: D06M11, D06Q1, C23C18