METHOD FOR PRODUCING AN OPEN-PORE MOLDED BODY WHICH IS MADE OF A METAL, AND A MOLDED BODY PRODUCED USING SAID METHOD

Abstract

A method for producing open-pored molded bodies made of a metal. The surface of the metal open-pored molded body being used as a semi-finished product, is coated with particles of the same metal with which the semi-finished product is made or with particles of a chemical compound of the metal the semi-finished product is made, wherein the compound or particles can be reduced or thermally or chemically decomposed in a thermal treatment. After the coating process, a thermal treatment in a suitable atmosphere is carried out, in which the particles are connected to the surface of the semi-finished product and/or adjacent particles such that the specific surface area of the obtained open-pore molded body is increased to at least 30 m.sup.2/l and/or at least by a factor of 5 in comparison to the starting material.

Claims

1. A process for producing open-pored molded bodies comprising a metal, wherein an open-pored shaped body comprising metal as a semifinished part is coated on its surfaces with particles of the same metal, of which the semifinished part is formed or is coated with particles of a chemical compound of the metal of which the semifinished part is made, which chemical compound can be reduced or thermally or chemically decomposed in a thermal treatment and being formed by particles of the respective metal obtained by chemical reduction or thermal or chemical decomposition; and the coating is followed by at least one thermal treatment in which the particles are joined via sinter necks or sinter bridges to the surface of the semifinished part and/or adjacent particles so that the specific surface area of the open-pored molded body obtained is increased to at least 30 m.sup.2/l and/or by at least a factor of 5 compared to the starting material of the uncoated metallic semifinished part, with a metal reducing atmosphere or an atmosphere suitable for the decomposition is maintained in the thermal treatment of a coated open-pored shaped body with particles of a reducible or thermally or chemically decomposable chemical compound of the metal of which the semifinished part is made, at least until the reduction or thermal or chemical decomposition of the chemical compound to form the metal is complete.

2. The process as claimed in claim 1, wherein the particles of a metal or the particles of a chemical compound of the metal are used as powder, powder mixture and/or suspension/dispersion.

3. The process as claimed in claim 1, wherein the application of the particles of the metal or the particles of the chemical compound of the metal as powder, powder mixture, suspension and/or dispersion is carried out by dipping, spraying, in a pressure-assisted manner, electrostatically and/or magnetically.

4. The process as claimed in claim 1, wherein an organic and/or inorganic binder is used in solution, suspension/dispersion or as a powder in order to improve the adhesion of particles.

5. The process as claimed in claim 1, wherein the application of particles of the metal or particles of the specified chemical compound of the metal is repeated a number of times.

6. The process as claimed in claim 1, wherein in the case of multiple coating with particles of the metal or particles of the chemical compound of the metal, when a binder is employed, the application of the binder is repeated a number of times.

7. The process as claimed in claim 1, wherein the application of a binder and the application of the particles of the metal or the particles of the chemical compound of the metal is carried out on different sides of the surface of the semifinished part using different amounts in order to obtain a different porosity, pore size and/or specific surface area.

8. The process as claimed in claim 1, wherein Ni, Fe, Cr, Al, Nb, Ta, Ti, Mo, Co, B, Zr, Mn, Si, La, W, Cu, Ag, Au, Pd, Pt, Zn, Sn, Bi, Ce or Mg is used as metal for the semifinished part and the particles to be applied or a chemical compound of Ni, Fe, Cr, Al, Nb, Ta, Ti, Mo, Co, B, Zr, Mn, Si, La, W, Cu, Ag, Au, Pd, Pt, Zn, Sn, Bi, Ce or Mg, is used as metal for the semifinished part and particles of a reducible, thermally or chemically decomposable compound of this metal.

9. The process as claimed in claim 1, wherein a semifinished part which has been obtained by electrochemical coating of an open-pored body of a polymeric material with the respective metal is used as semifinished part.

10. A coated and sintered open-pored molded body produced by a process wherein the molded body with metallic particles joined via sinter necks or sinter bridges to the surface of a semifinished part and/or the surface of adjacent particles has a specific surface area of at least 30 m.sup.2/1.

11. The coated and sintered open-pored molded body as claimed in claim 10, wherein pore size within the coated and sintered open-pored molded body corresponds to not more than 10 000 times the particle size used.

12. The coated and sintered open-pored molded body as claimed in claim 10, wherein not more than 3% by mass of oxygen is present in the material of the coated and sintered open-pored molded body.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0035] The invention will be illustrated below with the aid of examples.

Working Example 1

[0036] As semifinished part, an open-pored shaped body composed of silver, average pore size 450 m, having a porosity of about 95% and the dimensions 70 mm63 mm, thickness 1.6 mm (produced by electrolytic deposition of Ag on polyurethane foam), is subjected to a thermal treatment at a temperature of at least 400 C. in order to remove the organic components, especially those of the polyurethane.

[0037] To increase the specific surface area, a metallic powder, namely Ag metal powder having a particle size d.sub.50 in the range from 3 m to 9 m, is used in a total amount of 2 g.

[0038] Coating of the surface of the metallic open-pored shaped body as semifinished part is carried out using 0.6 g of stearamide wax having a particle size of <80 m and a 1% strength aqueous solution of polyvinylpyrrolidone having a volume of 6 ml as binder. The surface of the semifinished part is sprayed with the binder solution, including in the interior of pores, before the silver powder is applied to the surface coated with the binder.

[0039] Silver powder and the stearamide wax were mixed for 10 minutes using a Turbula mixer.

[0040] After this coating with binder, the open-pored coated shaped body was fixed in a vibration apparatus and sprinkled on both sides with the silver powder. The powder is distributed uniformly in the open-pore network by means of the vibration. The particles adhere only to the strut surface, so that the struts are completely covered with powder particles and the open porosity of the foam is retained. The procedure is repeated four times.

[0041] Subsequently, a further thermal treatment is carried out in a hydrogen atmosphere to effect binder removal and sintering. For this purpose, the furnace is heated up at a heating rate of 5 K/min. Binder removal commences at about 300 C. and is concluded at 600 C. and a hold time of about 30 minutes. The sintering process takes place in the temperature range from 550 C. to 850 C. at a hold time of from 1 minute to 60 minutes.

[0042] During the further thermal treatment, the Ag diffuses out of the powder particles into the strut material until the powder particles, via sinter necks or sinter bridges thereby formed, are firmly joined to the struts of the surface of the semifinished part.

[0043] After the further thermal treatment, the open-pored molded body consisted of 100% of silver. The porosity was about 94%.

[0044] The surface of the struts has a high roughness. The reason for this is that the applied powder particles are joined only via sinter necks or sinter bridges to the metallic support foam of the semifinished part, so that the original particle morphology is retained. The specific internal surface area (measured using the BET method) of the finished open-pored molded body could be increased from 10.8 m.sup.2/l initially (uncoated state) to 99.3 m.sup.2/l afterwards (coated state).

Working Example 2

[0045] An open-pored shaped body composed of silver as semifinished part having an average pore size of 450 m, a porosity of 95%, the dimensions 70 mm63 mm, thickness 1.6 mm, obtained by electrochemical coating of a porous foam composed of polyurethane, was subjected to a thermal treatment to remove the organic components, as in working example 1.

[0046] Surfaces of the semifinished part which had been freed of organic components were subsequently coated by spraying with a suspension having the following composition: [0047] 48% Ag.sub.2O metal oxide powder<5 m, [0048] 1.5% polyvinylpyrrolidone (PVP) binder [0049] 49.5% water as solvent [0050] 1% dispersant.

[0051] For this purpose, the pulverulent binder was firstly dissolved in water and then all other components were added and mixed in a Speedmixer for 230 seconds at 2000 rpm to give a suspension.

[0052] The semifinished part was sprayed with the prepared powder suspension a number of times on both sides by a wet powder spraying process. Here, the suspension is atomized in a spraying device and applied to surfaces on both sides of the semifinished part. The suspension is distributed uniformly in the porous network of the semifinished part by the exit pressure from the spray nozzle. The suspension adheres only to the strut surface, so that the struts are completely covered with the suspension and the open porosity of the semifinished part is largely retained. The semifinished part which has been coated in this way was subsequently dried in air at room temperature.

[0053] For binder removal, reduction and sintering, a thermal treatment was carried out under a hydrogen atmosphere and subsequently in a furnace. For this purpose, the furnace was heated up at a heating rate of 5 K/min. The reduction of the silver oxide commences at below 100 C. and is concluded at 200 C. and a hold time of about 30 minutes under hydrogen. The remaining binder removal and sintering process can then be carried out in an oxygen-containing atmosphere, e.g. air, in the temperature range from 200 C. to 800 C. at a hold time of from 1 minute to 180 minutes.

[0054] During the further thermal treatment, the silver oxide was firstly reduced to metallic silver, which is present in nanocrystalline form. As a result of the remaining binder removal and partial sintering of the then metallic silver particles onto the silver foam struts, the particles grow to form larger and more coarsely crystalline conglomerates, and secondly the Ag also diffuses out from the powder particles into the strut material until the powder particles are firmly joined via sinter necks or sinter bridges which form to the struts of the surface of the open-pored molded body.

[0055] After the further thermal treatment, a homogeneous open-pored molded body which is formed by 100% silver is present.

[0056] The porosity is about 93%.

[0057] The surface of the struts has a high roughness. The reason for this is that the applied powder particles are joined only via sinter necks/sinter bridges to the surfaces of the semifinished part, so that the original particle morphology is retained. The specific internal surface area (measured by the BET method) of the finished open-pored molded body was able to be increased from 10.8 m.sup.2/l initially (uncoated state) to 82.5 m.sup.2/l afterwards (coated state) by means of the process carried out.

Working Example 3

[0058] An open-pored shaped body composed of copper and having an average pore size of 800 m, a porosity of about 95%, the dimensions 200 mm80 mm, thickness 1.6 mm (produced by electrolytic deposition of Cu on PU foam), was used as semifinished part.

[0059] Electrolytic copper powder of the type FFL, having a dendritic form, an average particle size of <63 m and a mass of 20 g, was used as powder for coating surfaces of the semifinished part.

[0060] A 1% strength aqueous solution of polyvinylpyrrolidone having a volume of 20 ml was used as binder.

[0061] The semifinished part composed of copper was sprayed with the binder solution on both sides. The binder-coated semifinished part was subsequently fixed in a vibration apparatus and sprinkled on both sides with the copper powder. The powder is distributed in the porous network of the semifinished part by the vibration. The binder and powder coating was repeated three times, so that the pore space had been filled completely.

[0062] Binder removal and sintering were carried out in a thermal treatment under a hydrogen atmosphere. For this purpose, the furnace was heated up at a heating rate of 5 K/min. Binder removal commences at about 300 C. and is concluded at 600 C. and a hold time of about 30 minutes. Heating up is then continued up to a sintering temperature of 950 C. and this temperature was maintained for 30 minutes.

[0063] During the thermal treatment, the powder particles composed of copper sinter to one another and to the strut material until the powder particles are firmly joined via sinter necks or sinter bridges which form to the surface of the semifinished part, with a high porosity being retained and an increase in the specific surface area being achieved. The porosity of the open-pored molded body treated in this way is 54% and the specific surface area is 67 m.sup.2/I.

Working Example 4

[0064] An open-pored shaped body made of cobalt and having an average pore size of 580 m, a porosity of about 95%, the dimensions of 70 mm65 mm, thickness 1.9 mm (produced by electrolytic deposition of Co on PU foam), was used as semifinished part, Co metal powder having an average particle size of <45 m and a mass of 10 g and also stearamide wax having a particle size of <80 m and a mass of 0.1 g was used as powder, and a 1% strength aqueous solution of polyvinylpyrrolidone having a volume of 6 ml was used as binder.

[0065] Cobalt powder and stearamide wax were mixed for 10 minutes using a Turbula mixer.

[0066] The semifinished part composed of cobalt was sprayed on one side with the binder solution. It was subsequently fixed in a vibration apparatus and sprinkled on both sides with the cobalt powder. As a result of the vibration, the powder is uniformly distributed in the porous network of the semifinished part. The particles adhere only to the strut surface, so that the struts are completely covered with powder particles and the open porosity of the foam is initially retained. In a second step, the surface of the semifinished part is sprayed with binder solution on a first side to such a degree that the previously open pores are closed on one side by the binder, and the pore space close to the surface is completely filled by the subsequent further application of powder. On the opposite side of the semifinished part, only the struts are coated on the surface. As a result, the powder loading and thus the porosity in the foam is gradated from the first side to the opposite side of the semifinished part.

[0067] For binder removal and sintering, a thermal treatment was carried out in a hydrogen atmosphere. For this purpose, the furnace was heated up at a heating rate of 5 K/min. Binder removal commences at about 300 C. and is concluded at 600 C. and a hold time of about 30 minutes. This is followed by heating up to a sintering temperature of 1300 C. and this temperature maintained for 30 minutes.

[0068] During the thermal treatment, the Co diffuses out of the powder particles into the strut material of the semifinished part until the powder particles are firmly joined via sinter necks or sinter bridges which form both to the struts and also (in the completely filled regions) to one another.

[0069] The Co content of the finished open-pored molded body was 100%. The porosity is gradated over the total thickness of the molded body from the first side to the side located opposite the first and is about 54% on one side and about 93% on the other foam side. The specific surface area of the finished open-pored molded body is 69 m.sup.2/I.

Working Example 5 (Ni Expanded Metal Mesh+Ni PowderUniform Coating+Sintering

[0070] 1. Material

[0071] An open-pored nickel expanded metal mesh having a cell size of about 0.7 mm2 mm and the dimensions 75 mm75 mm, thickness about 1 mm (produced by stretching an originally 0.25 mm thick slotted Ni sheet) was used as semifinished part, Ni metal powder having an average particle size of <10 m and a mass of 8 g, a stearamide wax having an average particle size of <80 m and a mass of 0.2 g, was used as metal powder and a 1% strength aqueous solution of polyvinylpyrrolidone having a volume of 4 ml was used as binder.

[0072] Powder and stearamide wax were mixed for 10 minutes using a Turbula mixer.

[0073] The nickel expanded metal mesh was sprayed with the binder solution from two opposite sides. The mesh was subsequently fixed in a vibration apparatus and sprinkled on both sides with the nickel powder. As a result of the vibration, the nickel powder is uniformly distributed on the mesh. The particles adhere only to the mesh strut surface, so that the mesh struts are completely covered with powder particles and the open porosity of the expanded metal mesh is retained. The procedure was repeated five times.

[0074] Binder removal and sintering were carried out in a thermal treatment under a hydrogen atmosphere. For this purpose, the furnace was heated up at a heating rate of 5 K/min. Binder removal commences at about 300 C. and is concluded at 600 C. and a hold time of about 30 minutes. Heating up was then continued up to a sintering temperature of 1280 C. and this temperature was maintained for 30 minutes.

[0075] During the thermal treatment, the Ni diffuses out of the powder particles into the mesh strut material until the powder particles are firmly joined via sinter necks or sinter bridges which form to the mesh struts.

[0076] The open-pored molded body obtained in this way consisted of 100% of nickel.

[0077] The surface of the struts has a high roughness since the applied powder particles are joined only via sinter necks or sinter bridges to the support mesh of the semifinished part and to one another, so that the original particle morphology is largely retained. The applied high-porosity nickel layer on the struts has a thickness of from 1 m to 300 m. The porosity within the applied layer is 40%.