Method for producing an open-pored metal body having an oxide layer and metal body produced by said method

Abstract

An open-pored metal body, which is formed having a core layer (A) consisting of Ni, Co, Fe, Cu, Ag or an alloy formed having one of said chemical elements, wherein one of said chemical elements is present in the alloy at more than 25 at %, and a gradated layer (B) is formed on surfaces of the core layer (A), said gradated layer being formed by intermetallic phase or mixed crystals of Al, and a layer (C), which is formed having aluminum oxide, is formed on the gradated layer (B).

Claims

1. A process for producing an open-pored metal body, coating the surface of an open-pored semifinished part forming the core layer and consisting of Ni, Co, Fe, Cu, Ag or an alloy comprising one of these chemical elements, where one of these chemical elements is present in a proportion of more than 40 at % in the alloy, with pure aluminum powder or a powder of an aluminum alloy in which aluminum is present in a proportion of at least 40 at %; forming a gradated layer which comprises intermetallic phase or mixed crystals of Al on the surface of the open-pored semifinished part in a first heat treatment; and forming an aluminum oxide layer composed of pure α-Al.sub.2O.sub.3phase under oxidizing conditions from Al on the gradated layer in a subsequent second heat treatment of a temperature of at least 1200° C.

2. The process as claimed in claim 1, wherein the powder of an aluminum alloy in which aluminum and at least one of the metals selected from among Ni, Cu, Co, Mo, Fe, Ag, Mg, Si, Ti and W are present is used.

3. The process as claimed in claim 1, wherein the coating of the open-pored semifinished part surface, pure aluminum powder or a powder of an aluminum alloy is sprinkled on the surface of the open-pored semifinished part which has been coated with a binder in the form of a suspension or dispersion, with powder which has been sprinkled on and fixed to the surface by means of the binder, electrostatically or by means of action of magnetic force.

4. An open-pored metal body produced by a process as claimed in claim 1, wherein it comprises the core layer consisting of Ni, Co, Fe, Cu, Ag or an alloy comprising one of these chemical elements, where one of these chemical elements is present in a proportion of more than 25 at % in the alloy, and the gradated layer comprising of the intermetallic phase or mixed crystals of Al is formed on surfaces of the core layer and the layer of aluminum oxide composed of pure α-Al.sub.2O.sub.3 is formed on the gradated layer.

5. The open-pored metal body as claimed in claim 1, wherein the core layer comprises a metal foam, a mesh, a gauze, woven material, felt, lay-up or an open-pore body produced by an additive manufacturing process.

6. The open-pored metal body as claimed in claim 1, wherein the gradated layer or the aluminum oxide layer cover the surface of the core layer to an extent of at least 90%.

7. The open-pored metal body as claimed in claim 4, wherein the gradated layer has a thickness in the range from 1 μm to 50 μm or the aluminum oxide layer has a layer thickness in the range from 0.05 μm to 1 μm.

8. The open-pored metal body as claimed in claim 1, wherein a functional coating has been formed on the aluminum oxide layer.

Description

DESCRIPTION OF THE DRAWING

(1) The drawing shows:

(2) FIG. 1 a sectional view through an example of an open-pored metal body according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(3) Here, a core layer A, which can either be made of solid material or of struts which are hollow inside and comprises one of the metals Ni, Co, Fe, Cu, Ag or an alloy thereof, is provided with a gradated layer B. The oxide layer C is formed on the gradated layer B. This structure can form a support material A-C, with a functional coating D being able to be formed on the oxide layer C.

(4) It is possible to form an at least virtually closed oxide layer C which can function as controllable diffusion barrier and/or as thermal and electrical insulator between an active, functional coating D, applied on top and an underlying gradated layer B and also a metal core layer A of the semifinished part which ensure the oxidation and corrosion resistance of the structured support material under chemical and thermal stress, increase the mechanical stability of the open-pored, structured support material and make permanent, strong adhesion of an active, functional coating possible.

(5) Some metals, including Ni, Co, Fe, Cu and Ag, together with aluminum form intermetallic phases which can be converted by an oxidative treatment into pure aluminum oxide or mixed metallic oxides having a high proportion of aluminum oxide, which as coating on ductile metals reduce their elastic deformability, increase the mechanical stability, improve the adhesion of a functional coating D and as diffusion barrier controllable hinder or prevent the undesirable migration of elements from the metallic core layer and also the gradated layer into a functional coating formed thereon and can drastically improve the life of a metallic core layer A, a structured support material and a functional coating D. Especially in the field of electrochemical applications, for example the production of batteries and electrodes, the permanence of a high electrical conductivity and also thermal conductivity of the metallic core layer A and of the gradated layer B is advantageous. The oxide layer C can in this case function as insulator between the surface of the metallic core layer A, gradated layer B and a functional coating D. Furthermore, the oxide layer C passivates the metallic core layer A and the gradated layer B against corrosive media and thus prevents a decrease in the electrical and thermal conductivities as a result of corrosion and undesirable diffusion of elements from the metallic core layer A and the gradated layer B into a functional coating D formed thereon and also release of such elements into a surrounding medium.

(6) Some of the catalysts used in the chemical industry lose activity with an increasing period of operation as a result of various effects such as physical and chemical wear, dusting and leaching, i.e. the washing out of active metals in the reaction medium, so that they are consequently removed with the products and are no longer available for the catalysis. Apart from complete prevention of the undesirable migration of elements from the metallic core layer A and the gradated layer B by means of an oxide layer C functioning as diffusion barrier, their diffusivity for metal atoms and ions can be influenced by thickness, composition, crystal structure and density of the oxide layer C. This can be achieved by control of the chemical composition of the oxide layer C via the composition of the gradated phase in the gradated layer B, the thickness of the oxide layer C via duration, temperature and oxygen partial pressure in the oxidation process and also the phase composition via the temperature of the oxidation process. The metallic core layer A can comprise metals which represent the active component of a functional coating D. In this case, a desired, controlled migration of elements from the core layer A and the gradated layer B through the oxide layer C into the functional coating D allows compensation for the active component lost as a result of physical and chemical wear effects and makes a high catalytic activity combined with relatively long catalyst operating lives possible.

WORKING EXAMPLES

Working Example 1—Not According to the Invention

(7) An open-pored nickel foam having a cell size of the pores of 580 μm, a weight per unit area of 1000 g/m.sup.2 and a porosity of about 94%, a wall thickness of the struts between pores of 20 μm and a specimen size of 80 mm×80 mm, thickness 1.9 mm; produced by electrolytic deposition of Ni on PU foam and burning-out of the organic constituents, is used as semifinished part.

(8) Pure Al metal powder having an average particle size of <63 μm and a mass of 20 g is used for coating the semifinished part surface.

(9) As binder for the Al metal powder, a 1% strength aqueous solution of polyvinylpyrrolidone having a volume of 15 ml is produced.

(10) The nickel foam forming the semifinished part is sprayed on both sides with this binder solution. The foam is subsequently fixed in a vibration device and sprinkled on both sides with the Al metal powder. As a result of the vibration, this powder is uniformly distributed in the porous network of the foam. The procedure is repeated four times.

(11) Binder removal and sintering of the Al metal powder are carried out in a first heat treatment in a nitrogen atmosphere. For this purpose, a tube furnace is heated to 660° C. The coated semifinished part is brought from a zone having a temperature of 200° C. into a zone having a temperature of 660° C. for 2 s and then back into the cooler zone having a temperature of 200° C.

(12) During the heat treatment, most of the aluminum powder melts and reacts with the near-surface zones of the nickel foam struts. This forms a gradient of aluminum-rich and low-aluminum mixed crystals, phases with eutectic composition and also intermetallic phases of the material system Ni—Al with a concentration gradient between the aluminum-rich surface and the core surface region which is formed by pure nickel of the semifinished part material. The aluminum-rich phase NiAl.sub.3 with some additional either pure (100% by mass of Al) or eutectic (˜94% by mass of Al) aluminum regions remains on the surface. The proportion of aluminum decreases from the surface in the direction of the interior of the core layer A, in particular the struts of a metal foam. The layer thickness of the gradated layer B with the resulting alloy phase gradient is 15 μm. A pure Ni layer, which forms the core layer A and has a layer thickness of 10 μm, remains in the interior of the struts.

(13) In the next step, the aluminum-rich surface is utilized to produce a pure aluminum oxide covering layer C on the strut surface by oxidation, which covering layer C increases the thermal and chemical stability as a result of its passivating properties, decreases the diffusion of nickel ions on the surface and also improves the mechanical strength of the metallic semifinished part material which forms the core layer A. Oxygen partial pressure, duration and temperature of the oxidation are selected so that migration of aluminum atoms in the direction of the core layer A and the unwanted, complete oxidation down to the surface of the core layer A, in particular the struts of a metal foam, is prevented so as to rule out embrittlement of the material. The oxidation is carried out at a temperature of 635° C. in a preheated furnace over a time of 65 minutes using air as oxidant. During the oxidation, the thickness of the amorphous aluminum oxide layer C firstly increases to a critical thickness of 5 nm. After attainment of the critical thickness of the aluminum oxide layer C, cubic γ-Al.sub.2O.sub.3 crystals, which have a higher density and initially cover only part of the surface, are formed from the amorphous aluminum oxide phase. After an oxidative treatment for 65 minutes, a closed γ-Al.sub.2O.sub.3 layer C is formed on the surface of the struts which form the core layer A. The structured support material A-C is subsequently taken from the furnace and cooled at room temperature. This finally gives a 0.5 μm thick aluminum oxide layer C which contains predominantly γ-Al.sub.2O.sub.3 and has a density of 3660 kg/m.sup.3.

Working Example 2

(14) An open-pored cobalt foam having a cell size of the pores of 800 μm, having a weight per unit area of 1500 g/m.sup.2 and a porosity of about 89%, a wall thickness of the struts arranged between pores of 30 μm and a specimen size of 80 mm×80 mm, thickness 2.5 mm, is used as semifinished part. The semifinished part is produced by electrolytic deposition of Co on PU foam and subsequently burning-out of the organic constituents. Here, the struts form the core layer A.

(15) Al metal powder having an average particle size of <63 μm and a mass of 30 g was used for the coating.

(16) To form the surface coating of the semifinished part, a 1% strength aqueous solution of polyvinylpyrrolidone having a volume of 20 ml is prepared as binder.

(17) The cobalt foam of the semifinished part is sprayed on both sides with the binder solution. The semifinished part coated with the binder solution on the surfaces is subsequently fixed in a vibration device and sprinkled on both sides with the Al metal powder. As a result of the vibration, the Al metal powder is homogeneously distributed in the porous network of the semifinished part material. The procedure is repeated five times.

(18) Binder removal and sintering of the semifinished part coated with binder solution and Al metal powder is carried out in a nitrogen atmosphere. For this purpose, a tube furnace was heated to 665° C. The coated semifinished part is brought from a zone having a temperature of 200° C. into a zone having a temperature of 665° C. for 5 s and then back into the cooler zone having a temperature of 200° C.

(19) During the first heat treatment, most of the Al metal powder melts and reacts with the near-surface zones of the cobalt foam struts of the semifinished part forming the core layer A. Here, a gradated layer B, which consists of aluminum-rich and low-aluminum mixed crystals, phases having a eutectic composition and also intermetallic phases of the material system Co—Al corresponding to the concentration gradient, are formed at the surface starting out from the aluminum-rich surface to the pure cobalt core layer A of the semifinished part material. The aluminum-rich phase Co.sub.2Al.sub.9 with some additional either pure (100% by mass of Al) or eutectic (˜99% by mass of Al) aluminum regions remains at the surface. The proportion of aluminum decreases from the surface in the direction of the interior of the struts. The layer thickness of the surface region with the gradated layer B with resulting alloy phase gradients is 20 μm. A pure cobalt core layer A having an average layer thickness of the struts between pores of 20 μm remains in the interior of the struts.

(20) In the subsequent oxidation step, the aluminum-rich surface is utilized in a second heat treatment to form a pure aluminum oxide layer C on the strut surface by oxidation, which layer C increases the thermal and chemical stability due to its passivating properties, reduces the diffusion of cobalt ions at the surface and increases the mechanical strength of the metallic base material. Oxygen partial pressure, duration and temperature of the oxidation are selected so that migration of aluminum atoms in the direction of the cobalt core layer A and also the unwanted, complete oxidation through to the surface of the core layer A is prevented in order to rule out embrittlement of the material. The oxidation is carried out at 1050° C. in the preheated furnace over a time of 15 minutes using air as oxidant. During the oxidation, the thickness of the amorphous aluminum oxide layer C grows to a critical thickness of 5 nm. After attainment of the critical thickness, cubic γ-Al.sub.2O.sub.3 crystallites, which have a higher density and cover part of the strut surfaces, are formed from the amorphous aluminum oxide phase. With increasing duration of the oxidative treatment, a closed γ-Al.sub.2O.sub.3 layer is formed on the surface of the struts. After 15 minutes, a closed covering layer containing θ-Al.sub.2O.sub.3 as secondary phase and α-Al.sub.2O.sub.3 as main phase has been formed from the closed γ-Al.sub.2O.sub.3 layer as a result of the transitions of γ- to δ- to θ- and finally to α-Al.sub.2O.sub.3. The foam is subsequently taken from the furnace and cooled at room temperature. An aluminum oxide layer C which has a thickness of 0.5 μm-1 μm and contains, apart from a small proportion of θ-Al.sub.2O.sub.3, predominantly α-Al.sub.2O.sub.3, has a high density of up to 3990 kg/m.sup.3 and has, at 5 MPa, more than three times the compressive strength of a pure cobalt foam (1.5 MPa) is finally obtained.

Working Example 3

(21) An open-pored silver foam having a cell size of the pores of 450 μm, a weight per unit area of 2000 g/m.sup.2 and a porosity of about 88%, a wall thickness of the struts of which the core layer A is formed and which are arranged between pores of 50 μm and a specimen size of 75 mm×65 mm, thickness 1.7 mm, is used as semifinished part. The semifinished part is produced by electrolytic deposition of Ag on PU foam and subsequent burning-out of the organic constituents.

(22) A prealloyed AgAl metal powder consisting of 27% by weight of Al and 73% by weight of Ag and having an average particle size of <75 μm and a mass of 60 g was used for coating.

(23) To form the surface coating of the semifinished part, a 1% strength aqueous solution of polyvinylpyrrolidone having a volume of 30 ml is prepared as binder.

(24) The silver foam of the semifinished part is sprayed on both sides with the binder solution. The semifinished part which has been coated on the surfaces with the binder solution is subsequently fixed in a vibration device and sprinkled on both side with the prealloyed AgAl metal powder. As a result of the vibration, the prealloyed AgAl metal powder is homogeneously distributed in the porous network of the semifinished part material. The procedure is repeated eight times.

(25) Binder removal and sintering of the semifinished part coated with binder solution and prealloyed AgAl metal powder is carried out in a nitrogen atmosphere. For this purpose, a tube furnace is heated to 590° C. The coated semifinished part is brought from a zone having a temperature of 200° C. into a zone having a temperature of 590° C. for 10 s and then back into the cooler zone having a temperature of 200° C.

(26) During the first heat treatment, most of the prealloyed AgAl metal powder melts and reacts with the near-surface zones of the silver foam struts of the semifinished part forming the core layer A. Here, a gradated layer B, which consists of aluminum-rich and low-aluminum mixed crystals and also intermetallic phases of the material system Ag—Al according to the concentration gradient, is formed on the surface starting from the aluminum-rich surface through to the pure silver core layer A of the semifinished part material. The aluminum-rich phase Ag.sub.2Al remains at the surface. Virtually no pure (100% by mass of Al) aluminum regions were able to be observed because of the prealloying. The proportion of aluminum decreases from the surface in the direction of the interior of the struts. The layer thickness of the surface region with the gradated layer B with resulting alloy phase gradient is 25 μm. A pure silver core layer A having an average layer thickness of the struts between pores of 25 μm remains in the interior of the struts. In the subsequent oxidation step, the aluminum-rich surface is utilized in a second heat treatment to form a pure aluminum oxide covering layer on the strut surface by oxidation, which covering layer increases the thermal and chemical stability due to its passivating properties, reduces the diffusion of silver ions at the surface and increases the mechanical strength of the metallic base material. Oxygen partial pressure, duration and temperature of the oxidation are selected so that migration of aluminum atoms in the direction of the silver core layer A and also the unwanted, complete oxidation through to the surface of the core layer A, i.e. to the surface of the struts, is prevented so as to rule out embrittlement of the material. The oxidation is carried out at 900° C. in the preheated furnace over a period of 10 minutes using air as oxidant. During the oxidation, the thickness of the amorphous aluminum oxide layer grows to a critical thickness of 5 nm. After attainment of the critical thickness, cubic γ-Al.sub.2O.sub.3 crystallites, which have a higher density and cover part of the strut surfaces, are formed from the amorphous aluminum oxide phase. With increasing duration of the oxidative treatment, a closed γ-Al.sub.2O.sub.3 layer is formed on the surface of the struts. After 10 minutes, a closed covering layer containing both θ-Al.sub.2O.sub.3 and α-Al.sub.2O.sub.3 has been formed from the closed γ-Al.sub.2O.sub.3 layer as a result of the transitions from γ- to δ- to θ- and finally to α-Al.sub.2O.sub.3. The foam is subsequently taken from the furnace and cooled at room temperature. An aluminum oxide layer C which has a thickness of 0.5 μm-2 μm and contains θ-Al.sub.2O.sub.3 and α-Al.sub.2O.sub.3, has a high density of up to 3990 kg/m.sup.3 and, at 4 MPa, has more than four times the compressive strength of a pure silver foam (1 MPa) is finally obtained.