EP4177377A1 - Production of a self-supporting metal layer - Google Patents

Abstract

The invention relates to a method for producing a metal foil, comprising the steps of applying a metal layer to a substrate by means of aerosol deposition of a powder and at least partially or completely removing the substrate to obtain a foil. The method is particularly suitable for processing brittle metals such as tungsten into metal foils.

Description

The present invention relates to a method for producing a metal foil using the aerosol deposition method (ADM) and a metal foil, the metal foil comprising a relatively brittle metal or consisting of a relatively brittle metal.

Metal foils are used for a variety of applications. These range from household aluminum foils to photovoltaic solar modules, lithium-ion batteries, special batteries, heat exchangers, surface refinements, fuses, capacitors, transformers, electronics, medical technology, flexible circuit boards and special cables.

These metal foils usually consist of ductile metals such as nickel, copper, silver, gold, steel, lead, tin, titanium or alloys containing these metals. It is usually manufactured by repeated rolling until the desired thickness is reached. Films with a thickness of at least 5 μm can be produced in this way. Thermal post-treatment (annealing) is often necessary between individual rolling steps.

On the other hand, it has so far been a challenge to process brittle metals such as iridium, rhodium or tungsten thin foils, as these easily crack during rolling.

The object of the present invention was to provide a method with which metal foils can be produced from brittle metals, in particular metals with an atomic number >30, which in the pure state have a modulus of elasticity of at least 200 GPa.

In particular, it was an object of the invention to provide foils made from these brittle metals which have a maximum thickness of 200 μm.

At least one object of the invention is solved by the subject matter of the independent claims. The subjects of the subclaims relate to preferred embodiments.

1. a. Auftragen einer Metallschicht auf ein Substrat mittels Aerosol-Deposition eines Pulvers und
2. b. Zumindest teilweises oder vollständiges Entfernen des Substrats unter Erhalt einer Folie.

In a first aspect, the invention relates to a method for producing a metal foil, comprising the following steps:

1. a. Application of a metal layer to a substrate by aerosol deposition of a powder and
2. b. At least partial or complete removal of the substrate to obtain a film.

With the aid of the method according to the invention, foils can be produced from brittle metals which have a thickness in the range of 200 μm or less, in particular in the range of 10 μm-200 μm.

A film in the context of the invention can be understood as a homogeneous sheet made of very thin material, e.g. metal or plastic. A film usually extends much further in two spatial directions than in the third spatial direction.

The present invention relates to methods for producing metal foils and the metal foils obtainable therefrom.

Expansion of the metal foil is not further limited according to the invention. The metal foil may comprise ribbons, strips and expanses, e.g., rectangular in shape. In one possible configuration, the metal foil can be structured. For example, the film can be structured after step c), e.g. by lithographic methods, selective chemical or physical etching, embossing, folding or laser cutting, and combinations of these methods.

The metal foil contains or consists of a metal. Metals are to be understood as meaning those materials which have a metallic bonding character. This typically manifests itself in high electrical conductivity, which decreases with increasing temperature, and in high thermal conductivity. Depending on the surface condition, a metal optionally has a metallic luster. Within the scope of the invention, metals are to be understood as meaning elemental metals, alloys and intermetallic phases.

An elementary metal is a pure substance and has no other components apart from unavoidable impurities, e.g. from the manufacturing process. In particular, an elemental metal does not contain any intentionally added components. Alloys can be understood to mean mixtures of substances that contain one or more metals and have metallic properties. Optionally, an alloy may contain components that are not metals in elemental form, such as carbon, nitrogen, and oxygen.

In one possible embodiment, alloys can also include amorphous alloys which have solidified in the manner of glass and have no crystallinity. Exceptions to this are optionally present nanocrystalline phases that cannot be measured with XRD.

Examples of intermetallic phases are FeCr, TiAl, Ti3Al, TiAl3, NiAl, CuAl and FeAl. The intermetallic phases can optionally be present in different phases, such as alpha phases, beta phases, gamma phases, theta phases or sigma phases.

The metal foil preferably contains or consists of a metal. The metal preferably has an atomic number >20, in particular >30 in the periodic table. Furthermore, the metal preferably has a modulus of elasticity of at least 150 GPa, in particular at least 200 GPa. Within the scope of the invention, the modulus of elasticity can be determined by means of a tensile test in accordance with DIN EN ISO 6892-1:2020-06. In particular, the metal is a brittle metal. A brittle metal can be understood to mean, for example, a metal at the end of which the fracture occurs essentially without plastic deformation. Therefore, hardly any plastic deformation occurs before fracture and necking does not occur. The metal optionally has a melting temperature of at least 1800°C.

The metal contained in the metal foil is also preferably selected from the group consisting of Ir, Rh, Os, Ru, Pt, Pd, W, Mo, Cr, Zr, Hf, Ti, Re, Zn and Si and combinations of the aforementioned elements. Silicon (Si) is not a metal in the narrower sense, but a semimetal and is preferably also included in the invention. However, it can be contained in particular in alloys with the other elements of the group.

In step a), a metal layer is applied to a substrate by aerosol deposition of a powder.

The aerosol deposition process is a coating process.

A coating system is used for application with aerosol deposition. This coating system contains an aerosol generator in which the powder used is converted into an aerosol as described in step b). In addition, the ADM coating system contains a coating chamber in which the coating process takes place. A vacuum in the preferred range of 60 mbar to 1066 mbar prevails in the aerosol generator, particularly during the process. A pressure in the preferred range of 0.2 mbar - 20 mbar prevails in the coating chamber, particularly during the process. The pressure in the coating chamber is lower than the pressure in the aerosol generator. Typically, the pressure difference between the aerosol generator and the coating chamber is in the range of 200 mbar - 500 mbar. The powder is transported from the aerosol generator through a nozzle into the coating chamber via a process gas. The particles are accelerated due to the resulting pressure difference between the aerosol generator and the coating chamber, in particular to a speed in the range of 100 m/s - 600 m/s, and are deposited on the surface of the metal body. The particles deform plastically or break up as a result of the impact into fragments, particularly in the nm range, and form a dense and well-adhering layer. The process gas used can be selected from inert gases, oxygen, air or combinations thereof. The inert gas can preferably be at least one gas selected from the group consisting of helium (He), argon (Ar) and nitrogen (N ₂ ) or combinations thereof. The substrate is preferably moved with an XY table. Alternatively or additionally, it is possible for a nozzle to be moved over the substrate to be coated. A combination of both, i.e. a movement of the table and a nozzle in the XY direction against each other, is also possible. ADM is a cold coating process, ie the deposition preferably takes place at a maximum temperature of 50°C.

In contrast to this, with cold gas spraying , the application is carried out with the help of a carrier gas at several 100°C, with which 50 - 90 µm particles of ductile materials are thrown onto the surface. In contrast to aerosol deposition, the particles do not break apart, but the particle morphology is essentially preserved. So with 50 µm particles no mean layer thicknesses >>50 µm are possible.

An advantage of the aerosol deposition process is that small foils can be produced without significant loss of material.

The powder used for the aerosol deposition preferably has a particle size distribution with a dso value in the range of 0.1 μm-150 μm. Furthermore, the particle size distribution can have a d ₉₀ value of 125 μm or less. The particle size distribution within the scope of the invention can be determined, for example, by means of laser diffraction in accordance with ISO 13320:2009. The powder can have a monomodal or a multimodal particle size distribution.

The substrate to which the metal layer is applied can have or consist of a material that is selected from the group consisting of metals, semimetals, ceramics, glasses and polymers. Examples of substrates are steel plates, steel foils, silicon wafers, alumina ceramics, mica substrates, silicate glass plates and polyimide substrates

The surface of the substrate to which the metal layer is applied is preferably flat. The surface of the substrate on which the metal layer is applied preferably has a roughness R _z with a value in the range of at most 10 μm, in particular if the surface is planar. The roughness can be measured using the profile method according to DIN EN ISO 4287:2010-07. A roughness that is as small as possible can be advantageous since the substrate can then be removed more easily from the metal layer.

In one possible embodiment, the substrate has a three-dimensional structure, in particular in the form of elevations and depressions. The elevations and depressions preferably have an extent of at least 100 μm in at least one spatial direction. In one possible embodiment of the invention, the substrate on which the metal layer is applied has a separating layer. The separating layer can serve to reduce the adhesion between the substrate and the metal layer. The separating layer can be, for example, a film with a thickness of at most 1 μm. The release layer may comprise a material selected from the group consisting of oxides (oxides (e.g. quartz, glass) polymers, fats and graphite.

Within the scope of the invention, when an application to the substrate is mentioned, it also includes that there is a separating layer on the substrate.

The metal layer applied to the substrate preferably has an average thickness of at least 5 μm, in particular at least 10 μm and very particularly preferably at least 20 μm. At the same time, the metal layer applied to the substrate preferably has an average thickness of at most 300 μm, in particular, in particular at most 200 μm and very particularly preferably at most 100 μm.

Furthermore, the metal layer preferably has a relative density of at least 70%, in particular at least 80% and particularly preferably at least 95%. Alternatively, the relative density of the metal layer can be at least 99%.

The purity of the metal layer preferably corresponds to the purity of the powder that was applied by means of aerosol deposition. For example, the purity of the metal layer can be at least 99.5%, in particular at least 99.95% and very particularly preferably at least 99.995%. Typically, the purity ranges from 99.9% to 99.999%.

The substrate and the metal layer together form a layered composite. A separating layer can optionally be arranged in the layer composite between the substrate and the metal layer.

In step b) the substrate is removed to obtain a metal foil.

The removal of the substrate can be partial or complete. When the substrate is partially removed, at least a portion of the metal foil cannot be in contact with the substrate.

The removal can take place, for example, by detaching or dissolving the substrate. The substrate can preferably be detached from the metal layer by heating the layered composite of substrate and metal layer. Due to the different linear thermal expansion coefficients of the substrate and the metal layer, the substrate can flake off the metal layer and subsequently be removed. This thermal detachment can be achieved in particular when the coefficient of linear thermal expansion alpha (x 10 ^-6 K ^-1 ) of the metal layer and the substrate differ from one another. The linear thermal expansion coefficient can be determined according to DIN 51045-1:2005-08. The difference in the linear thermal expansion coefficients is preferably at least 20%, in particular at least 50% and particularly preferably at least 100%. In another preferred embodiment, the difference in the linear thermal expansion coefficients is at least 2×10 ^-6 K ^-1 , in particular at least 5×10 ^-6 K ^-1 and very particularly preferably at least 20×10 ^-6 K ^-1 . In one example, the substrate can be an alumina substrate with a linear thermal expansion coefficient of 8.1×10 ^-6 K ^-1 and the metal layer can be tungsten and have a linear thermal expansion coefficient of about 4.4×10 ^-6 K ^-1 .

In another embodiment, the substrate can be removed by grinding and polishing. In particular, the substrate can be ground off completely.

Alternatively, the substrate can be removed by etching, particularly wet chemical etching, or physical etching, e.g., plasma etching.

In one possible embodiment, in which a separating layer is arranged on the substrate during the aerosol deposition, this separating layer can also be partially or completely removed by removing the substrate.

A metal foil is obtained by removing the substrate.

The metal foil is preferably a free-standing metal foil. This means that the metal foil is not placed on any type of support. In other words, both side faces of the metal foil can be freely accessible.

In a preferred embodiment, the metal foil has a thickness in the range of at least 10 μm, in particular at least 20 μm and of at most 300 μm, in particular at most 200 μm. The thickness of the foil preferably essentially corresponds to the thickness of the metal layer from step a). Optionally, the thickness of the foil can be thinner than the metal layer when the foil is post-processed.

Furthermore, the metal foil preferably has a relative density of 70% or more, in particular at least 90% or even at least 99%. The metal foil preferably has no gas inclusions.

Following step b), the metal foil can optionally be post-treated. Possible post-treatment processes include temperature treatments, rolling, grinding, polishing, pressing, in particular uniaxial pressing, cold isostatic pressing, or hot isostatic pressing.

In the event that a separating layer is arranged on the substrate, components of this separating layer can optionally remain on the metal foil when the substrate is removed. These components of the separating layer are preferably removed from the metal foil.

In a second aspect, the invention relates to a metal foil, in particular producible by a method as described herein, the metal contained in the metal foil having an atomic number >30 in the periodic table and a modulus of elasticity of at least 200 GPa, characterized in that the Film has a thickness in the range of 10 - 300 microns.

In one embodiment, the foil can be a precious metal foil. The metal foil can particularly preferably be an iridium foil, a rhodium foil, a tungsten foil, an osmium foil, a ruthenium foil, a platinum foil, a palladium foil, a molybdenum foil, a chromium foil, a zirconium foil, a hafnium foil, a titanium foil, a rhenium foil, a zinc foil or a silicon foil.

The purity of the metal foil is preferably in the range of the purity of the particles used and can be, for example, in the range from 99.5 to 99.9995%.

The use of the metal foil according to the invention is not further restricted. Various applications are conceivable. For example, the metal foil can be used for electromagnetic shielding, as a catalyst support or as a lining for apparatus in chemical equipment construction.

In one aspect, the invention relates to a sensor or a radiation protection device containing a metal foil according to the invention. In particular, the metal foil can be a rhodium foil used for mammography.

In a further embodiment, the metal foil can be a tungsten foil for medical technology or nuclear medicine. In particular, the radiation protection for gamma radiation provided by a tungsten foil can be about 50% higher compared to lead. This can result in particular advantages for imaging.

example

In one example, tungsten powder (purity 99.9%) with a particle size d ₅₀ =10 μm was deposited as a coating material on an aluminum substrate (EN AW-5005) by means of aerosol deposition. The process parameters are given in Table 1. <i>Table 1</i> coating material Tungsten (99.9%) substrate material Aluminum (EN AW-5005) substrate roughness R _z = 1.5 µm Pressure in coating chamber 10 mbar Pressure in Aerosol Generation Chamber 500 mbar carrier gas helium

A tungsten layer with a layer thickness of 23 μm was produced. The composite of tungsten layer and aluminum substrate was then placed in hydrochloric acid (36%) for 10 minutes. After the aluminum substrate had dissolved, a self-supporting tungsten foil could be removed from the solution.

Claims (15)

Method for producing a metal foil, comprising the following steps: a. Application of a metal layer to a substrate by aerosol deposition of a powder and b. at least partial or complete removal of the substrate to obtain a film. Method according to any one of the preceding claims, wherein the removal of the substrate from the metal layer can be selected from detaching and dissolving, as well as combinations thereof. Method according to one of the preceding claims, wherein the substrate is selected from metals, ceramics, glasses and plastics. Method according to one of the preceding claims, wherein the metal foil has a thickness in the range of at least 10 µm, in particular at least 20 µm and at most 300 µm, in particular at most 200 µm. Method according to one of the preceding claims, in which the roughness of the substrate R _z has a value in the range of at most 10 µm. Method according to one of the preceding claims, wherein the linear thermal expansion coefficients α (x 10 ^-6 K ^-1 ) of the metal layer and the substrate differ by at least 20%. A method according to any one of the preceding claims, wherein the metal foil has a relative density of 70% or more. A method according to any one of the preceding claims, wherein the metal foil is gas free and has a relative density in the range of 99.5% or more. Method according to one of the preceding claims, wherein the metal contained in the metal foil has an atomic number >20 in the periodic table and a Young's modulus of at least 150 GPa. A method according to any one of the preceding claims, wherein the metal has a melting temperature of at least 1800°C. Method according to any one of claims 12 - 14, wherein the metal is selected from the group comprising Ir, Rh, Os, Ru, Pt, Pd, W, Mo, Cr, Zr, Hf, Ti, Re, Zn and Si and combinations of the above Elements. Metal foil, in particular producible by a method according to one of claims 1 - 11, wherein the metal has an atomic number >20 in the periodic table and a modulus of elasticity of at least 150 GPa, characterized in that the foil has a thickness in the range of 10 - 300 µm. Metal foil according to claim 12, wherein the metal foil is a free-standing metal foil which is at least not completely arranged on a support. Metal foil according to one of Claims 12 or 13, the metal having a purity of at least 99.5% by weight, in particular at least 99.95% by weight. Sensor or radiation protection device containing a metal foil according to one of Claims 12 to 14.