US20200263306A1

US20200263306A1 - Method for producing an open-pore molded body which is made of a metal, and a molded body produced using said method

Info

Publication number: US20200263306A1
Application number: US16/648,028
Authority: US
Inventors: Tilo BUETTNER; Gunnar Walther; Hans-Dietrich BOEHM; Thomas Weissgaerber; Bernd KIEBACK; Christian Immanuel Müller; Robin Kolvenbach; Lars Torkuhl
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV; Alantum Europe GmbH
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV; Alantum Europe GmbH
Priority date: 2017-09-06
Filing date: 2018-09-14
Publication date: 2020-08-20
Also published as: JP2020534433A; KR20200127966A; DE102017216569A1; EP3684532A1; CA3076512A1; RU2020111275A; CN111432961A; WO2019057624A1

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 m2/l and/or at least by a factor of 5 in comparison to the starting material.

Description

BACKGROUND OF THE INVENTION

The invention relates to a process for producing an open-pored molded body or an shaped body comprising a metal and a molded body produced by the process.
Coating porous metallic molded bodies on their surface in order, in particular, to improve the properties is known. For this purpose, use is customarily made of pulverulent materials which are applied by means of a binder or a suspension to surfaces of the molded body and organic constituents are removed in a heat treatment and a coating or a surface region which has a different chemical composition than the material of which the shaped body was made can then be formed on surfaces of the shaped body at elevated temperatures.
The specific surface area of a shaped body can also be increased by means of these known possibilities, but this was possible to only a limited extent by means of the known possibilities.
However, very large specific surface areas are advantageous for many industrial applications, and is very desirable in, for example, catalytically assisted processes, filtration or in electrodes in electrochemical applications.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide open-pored molded bodies which are composed of a metallic material and have an increased specific surface area.
This object is achieved according to the invention by a process having the features of the independent claim which relates to a molded body produced by the process. Advantageous embodiments and further developments can be realized by means of the features in the dependent claims.
In the invention, open-pored bodies composed of a metallic material are used as a semifinished part. These can be a metal grid, a metal mesh, a woven metal fabric, a metal foam, a metal wool or a semifinished part comprising metallic fibers.
However, the semifinished part can also be an open-pored shaped body in which a polymer material has been electrochemically coated with a metal. A semifinished part produced in this way can be subjected to a thermal treatment in which the organic and volatile constituents of this polymer are removed as a result of pyrolysis. However, this removal of the organic constituents of a polymer can also occur later in a simultaneous removal of other organic or volatile components, which will be discussed in more detail below. In one embodiment of the invention, this thermal treatment is preceded or followed by
coating of the open-pored body with metallic particles composed of the same metal material of which the open-pored semifinished part is made. Here, the particles should also be introduced into the interior of the shaped body, i.e. into the pores or voids of the semifinished part.
In a further embodiment of the invention, particles of a chemical compound of the chemical element present in the open-pored shaped body as semifinished part are applied by coating before or after this thermal treatment. Said particles consist of a chemical compound which can be converted in a thermal treatment by chemical reduction or thermal or chemical decomposition into the respective chemical element of which the semifinished part is made.
The metallic particles of the same metal material from which the open-pored semifinished part has been produced or the particles of a chemical compound of the chemical element which can be converted into the chemical element of which the open-pored molded body as semifinished part has been made can be used as powder, as powder mixture, as suspension or as dispersion for the coating operation. Coating of the surface of the semifinished part with a powder, a powder mixture and/or a suspension/dispersion can be carried out by dipping, spraying, in a pressure-assisted manner, electrostatically and/or magnetically.
In further alternatives according to the invention, the powders, powder mixtures, suspensions or dispersions used for coating the open-pored semifinished part can contain not only metallic particles or particles of a chemical compound of a metal but also an inorganic and/or organic binder which is mixed in finely divided form as a solid powder into the powder, the powder mixture, the suspension or dispersion or is present dissolved in a liquid phase of a solution, a suspension/dispersion of metallic particles or particles of a chemical compound of a metal.
Coating of the surface of the semifinished part with a binder in the form of a solution or a suspension/dispersion can be effected by dipping or spraying.
The open-pored semifinished part which has been wetted with binder is subsequently coated with a powder or a powder mixture of metallic particles.
The distribution of powder particles on surfaces which have been wetted with the liquid binder and also the adhesion of the particles to the surface can be improved by action of mechanical energy, in particular vibration.
The application of particles as powder, powder mixture and/or suspension/dispersion can be repeated a number of times, preferably at least three times, particularly preferably at least five times. This also applies to the vibration to be carried out in each case and optionally the application of a binder.
Coating of the surface of the semifinished part can also be carried out before the thermal treatment in which the organic constituents of the polymeric material with the aid of which the semifinished part has been produced are removed. After application of the particle-containing material, a thermal treatment in which organic and volatile constituents of the polymeric material and at the same time any binder used are removed is carried out.
After thermal treatment and application of particles, sintering in which sinter necks or sinter bridges between the metal particles or from metallic particles obtained by thermal or chemical decomposition, e.g. a chemical reduction, to the metallic surface of the open-pored metallic molded body are formed is carried out.
Here, the specific surface area of the open-pored molded body which has been coated and sintered in this way should be increased to at least 30 m²/l but at least by a factor of 5 compared to the starting material of the uncoated metallic shaped body as semifinished part.
Here, the porous basic framework having a pore size in the range from 450 μm to 6000 μm and a specific surface area of 1 m²/l 30 m²/l should be filled with particles (particle size d₅₀in the range from 0.1 μm to 250 μm), de-pending on the application either from one side (porosity gradient) or completely or the struts of the porous metallic shaped body should have been coated on the surface.
Coating with particles can be carried out using different amounts on different sides of the surface, in particular on surfaces of the semifinished part which are arranged opposite one another, in order to obtain a different porosity, pore size and/or specific surface area in each case. This can, for example, be achieved by a different number of applications of particles as powder, powder mixture or in suspension/dispersion, with or without use of binder, on the surfaces arranged on different sides. A gradated formation of a molded body produced according to the invention can also be achieved in this way.
The pore size within the applied particle layer of the coated and sintered open-pored molded body should correspond to not more than 10 000 times the particle size used. This can be additionally influenced by the maximum sintering temperature and the hold time at this temperature since mass transfer by diffusion and thus sintering, which is associated with a decrease in the pore volume, is promoted with increasing temperature and hold time.
The material of which the molded body produced according to the invention is made should contain not more than 3% by mass, preferably not more than 1% by mass, of O₂. Preference is for this purpose given to an inert or reducing atmosphere while carrying out the thermal treatment for removing organic components, the chemical reduction which is optionally to be carried out and/or the sintering.
For a thermal or chemical decomposition, a suitable atmosphere should be selected in the thermal treatment utilized for this purpose. This can in the case of a thermal decomposition be an inert atmosphere, for example an argon atmosphere. In the case of a reduction, it is possible to employ, for example, an atmosphere of hydrogen.
For a chemical decomposition by means of oxidation, atmospheres containing oxygen, fluorine, chlorine, any mixtures of these gases and also any mixtures with inert gases, for example nitrogen, argon or krypton, are particularly useful.
In the case of a chemical decomposition, metal cations can be reduced to form elemental metals. It is, however, possible to oxidize the anion constituent. A chemical decomposition of a compound of relatively noble metals to give the elemental metals (Au, Pt, Pd) in air, i.e. a comparatively oxidizing atmosphere, is also conceivable. Disproportionations according to the illustrative equation: 2 Gel<->Ge (s)+Gel (g) are also possible for aluminum, titanium, zirconium and chromium. It is also possible to use crystalline, metal-organic complexes or salts thereof in which the metal center is already in the oxidation state 0.
It is also possible to employ such an open-pored molded body produced according to the invention in the field of (i) filtration, as (ii) catalyst (e.g. in the synthesis of ethylene oxide using an Ag foam catalyst coated with Ag particles), as (iii) electrode material or as (iv) support for a catalytically active sub-stance.
Increasing the specific surface area leads, in the case of application (i), to a better filtration performance since adsorption tendency and uptake capacity are significantly increased.
In application (ii), the increase in the specific surface area leads to a greater than proportional increase in the catalytic activity since not only does the number of active centers increase but the surface also has a distinctly faceted structure. The resulting increased surface energy additionally leads to a significant increase in the catalytic activity compared to the unfaceted surface of the open-pored starting shaped body.
In application case (iii), the increase in the specific surface area likewise leads to an increase in active centers, which in combination with the faceted structure of the surface leads to a significant reduction in the electric overvoltage compared to commercial electrodes (e.g. nickel or carbon). As specific application, mention may also be made of electrolysis, e.g. using Ni or Mo foam coated with Ni particles or Mo particles. In this application in particular, it is also advantageously possible to use sintered metallic open-pored molded bodies coated on one side with metallic particles since in this case the gradation of the pore size ensures that the gas bubbles are transported away well.
In the case of application (iv), the increase in the specific surface area leads to better adhesion of the active component, e.g. a catalytic washcoat, to the support surface, which significantly increases the mechanical, thermal and chemical stability of a catalyst material.
Suitable metals for molded bodies produced according to the invention are: 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. Particles of these elements, corresponding to the respective chemical element of which the semifinished part is made, can accordingly be used in the process of the invention for coating a semifinished part.
As chemical compounds of the metals Ni, Fe, Cr, Al, Nb, Ta, Ti, Mo, Co, B, Zr, Mn, Si, La, W, Cu, Ag, Au, Pd, Pt, Zn, Sn, Bi, Ce, Mg, V which can be converted by thermal or chemical decomposition in a thermal treatment into particles of the respective metal it is possible to use, in particular, their oxides, nitrides, hydrides, carbides, sulfides, sulfates, phosphates, fluorides, chlorides, bromides, iodides, azides, nitrates, amines, amides, metal-organic complexes, salts of metal-organic complexes or decomposable salts for the material formed with particles, with which the surface of the open-pored shaped body present as semifinished part is to be coated in the second alternative according to the invention. Particularly suitable chemical compounds are chemical compounds of: Ni, Fe, Ti, Mo, Co, Mn, W, Cu, Ag, Au, Pd or Pt.
In the thermal or chemical decomposition of a chemical compound to give the respective metal, an atmosphere suitable for the decomposition, which can be inert, oxidizing or reducing, is maintained until the thermal or chemical decomposition of the chemical compound into the metal has occurred. For the chemical reduction of a chemical compound to the respective metal, the thermal treatment which is to lead to the chemical reduction can preferably be carried out in a reducing atmosphere, in particular a hydrogen atmosphere, for at least some of the time until the chemical reduction has been carried out.
Porosity, pore size and specific surface area can be substantially influenced by the morphology of the particles used for the coating. To achieve a high specific surface area and a finely porous structure, particles having a small size and a dendritic shape, e.g. electrolyte powders, are advantageous. As a result of their irregular geometry which does not allow a gap-free arrangement, adjacent particles form voids which are partially connected to give channels between contact points and particle bodies. Furthermore, an additional micropore space left behind by the volatile component is formed in the thermal decomposition or chemical decomposition when using particles of a chemical compound. The greater the proportion of the volatile component of the chemical compound, the higher the proportion of the micropore space in the total pore volume. The use of an oxide having a high oxidation state and consequently a high proportion of oxygen is therefore advantageous for a coating with metal oxide particles. Since the sintering activity of structures increases with increasing specific surface area, the material-dependent sintering temperature is chosen so as to be just high enough for the particles to sinter to one another and to the semifinished part in a mechanically stable manner without the fine pores being significantly densified.

DETAILED DESCRIPTION OF THE INVENTION

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

Working Example 1

As semifinished part, an open-pored shaped body composed of silver, average pore size 450 μm, having a porosity of about 95% and the dimensions 70 mm×63 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.
To increase the specific surface area, a metallic powder, namely Ag metal powder having a particle size d₅₀in the range from 3 μm to 9 μm, is used in a total amount of 2 g.
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 μm 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.
Silver powder and the stearamide wax were mixed for 10 minutes using a Turbula mixer.
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.
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.
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.
After the further thermal treatment, the open-pored molded body consisted of 100% of silver. The porosity was about 94%.
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 m²/l initially (uncoated state) to 99.3 m²/l afterwards (coated state).

Working Example 2

An open-pored shaped body composed of silver as semifinished part having an average pore size of 450 μm, a porosity of 95%, the dimensions 70 mm×63 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.
Surfaces of the semifinished part which had been freed of organic components were subsequently coated by spraying with a suspension having the following composition:

- 48% Ag₂O metal oxide powder<5 μm,
- 1.5% polyvinylpyrrolidone (PVP) binder
- 49.5% water as solvent
- 1% dispersant.

For this purpose, the pulverulent binder was firstly dissolved in water and then all other components were added and mixed in a Speedmixer for 2×30 seconds at 2000 rpm to give a suspension.
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.
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.
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.
After the further thermal treatment, a homogeneous open-pored molded body which is formed by 100% silver is present.
The porosity is about 93%.
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 m²/l initially (uncoated state) to 82.5 m²/l afterwards (coated state) by means of the process carried out.

Working Example 3

An open-pored shaped body composed of copper and having an average pore size of 800 μm, a porosity of about 95%, the dimensions 200 mm×80 mm, thickness 1.6 mm (produced by electrolytic deposition of Cu on PU foam), was used as semifinished part.
Electrolytic copper powder of the type FFL, having a dendritic form, an average particle size of <63 μm and a mass of 20 g, was used as powder for coating surfaces of the semifinished part.
A 1% strength aqueous solution of polyvinylpyrrolidone having a volume of 20 ml was used as binder.
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.
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.
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 m²/I.

Working Example 4

An open-pored shaped body made of cobalt and having an average pore size of 580 μm, a porosity of about 95%, the dimensions of 70 mm×65 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 μm and a mass of 10 g and also stearamide wax having a particle size of <80 μm 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.
Cobalt powder and stearamide wax were mixed for 10 minutes using a Turbula mixer.
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.
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.
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.
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 m²/I.

Working Example 5 (Ni Expanded Metal Mesh+Ni Powder→Uniform Coating+Sintering

1. Material
An open-pored nickel expanded metal mesh having a cell size of about 0.7 mm×2 mm and the dimensions 75 mm×75 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 μm and a mass of 8 g, a stearamide wax having an average particle size of <80 μm 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.
Powder and stearamide wax were mixed for 10 minutes using a Turbula mixer.
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.
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.
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.
The open-pored molded body obtained in this way consisted of 100% of nickel.
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 μm to 300 μm. The porosity within the applied layer is 40%.

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 m²/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 m²/1.

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.