Patent Application
20090011315
The invention relates to a composite element comprising at least one thin film that consists of at least two different layers of an oxide-ceramic material, an oxide-ceramic and metallic material or a metallic material and comprising an essentially flat substrate that supports the thin film. Furthermore, the invention relates to a method for structuring the substrate and to uses of the composite element with the thin film.
Thin films, in particular electrically conducting thin films of ceramic and/or metallic materials are currently gaining in importance the whole time. The thin films generally consist of a number of layers, in particular three to five, the material and/or the morphology of the individual layers generally being different. The thin film is deposited in layers on the substrate, customary thin-film techniques being used, for example chemical vapor deposition, pulsed laser vapor deposition, sol-gel methods, in particular rotational coating, or spray pyrolysis. Furthermore, the thin film may be applied to the substrate as a whole or layer by layer as such. After or during the application, the layers or the thin film as a whole is or are annealed in a single-stage or multi-stage process, to obtain a partially or fully crystalline microstructure. Multilayer thin films are also referred to as laminates.
U.S. Pat. No. 6,896,989 B2 describes thin films that are applied to a substrate, consist of a number of layers and can be used as electrodes and solid electrolyte in fuel cells. Arranged between these functional layers are further layers, also made of the material of the electrode. Optionally, additional layers of different materials may also be added. According to this patent specification, the individual layers of the thin film are deposited by methods that are known per se, such as RF (radio frequency) sputtering, PVD (physical vapor deposition), CVD (chemical vapor deposition) and electrophoresis.
The known substrates are unable to satisfy requirements for thin films that have a number of layers and correspondingly complex properties of their mechanical structure (crack formation, inadequate service life).
The present invention is based on the object of providing a composite element that is stable mechanically and under elevated temperatures comprising at least one thin film and a substrate of the type mentioned at the beginning, so that miniaturized electrochemical devices produced with the composite element have a high power density and good material compatibility. In particular, a substrate which mitigates mechanical stresses and is suitable for complex multilayer structures is to be proposed.
The object is achieved according to the invention with respect to the composite element by the substrate consisting of a ceramicizable glass, a glass ceramic, a mixed form or an intermediate state of the two.
The combination according to the invention of multilayer thin films and ceramicizable glass achieves the effect that the substrate softens to a certain extent at the temperatures of the production process and the stresses occurring in the multilayer thin film are reduced. Because, in the arrangement according to the invention, the substrate adapts itself to the thin film on account of the softening, the stress-induced deformations of the layers do not lead to cracks and ruptures.
Special embodiments and further developments of the invention are the subject of dependent patent claims.
Hereafter, the expression glass or glass substrate always includes a ceramicizable glass, a glass ceramic, a mixed form or an intermediate state of the two.
The substrate is preferably a light-sensitive, structurable glass which can be etched. The light-sensitive glass FOTURAN from the MIKROGLAS company is particularly suitable. This special glass is generally pore-free, but may also be formed such that it is completely porous or porous over certain zones. Further advantageous properties of this glass are the chemical, dimensional and thermal stability and the homogeneity.
The substrate that is used according to the invention has a coefficient of thermal expansion lying in the range of the ceramic layers of the thin film that is applied. This range lies in practice at (5-20)·10−6K−1. A composite element with a coefficient of thermal expansion of the thin film in the range of (8-15)·10−6K−1 and a coefficient of thermal expansion of the glass in the range of (8-10)·10−6K−1 is expedient. Neighboring or overlapping coefficients of thermal expansion have obvious advantages, in particular with respect to the stability of the composite under shock-like temperature changes. With greatly differing coefficients of thermal expansion, these may lead to defects, deformations and destruction. Substrates based on silicon, for example, have a coefficient of thermal expansion that is three to five times lower than the ceramic materials of the thin film, for which reason these substrates cannot be used in the present case.
Sufficient mechanical stability is imparted to the thin film by the thickness of a glass substrate supporting it usually corresponding to at least approximately five times, preferably at least approximately ten times, the total layer thickness of the thin film. The glass substrate may be formed such that it is flexible, for example as a sheet, or rigid, for example as a plate. Both embodiments of the substrate can be impermeable, porous over the entire surface area or parts thereof and/or have holes or channels that can be configured as desired, which is referred to as a structured substrate. At least parts of the porous regions and the holes or channels are covered by the thin film, which in this function is referred to as a membrane. The channels also serve for fluid distribution; they may also be formed as grooves that pass only part of the way through the substrate. According to a special embodiment of the invention, the thin film may also be formed such that it is as thick as or thicker than the substrate. For the sake of simplicity, any ceramic layer composite is referred to in the present case as a thin film.
The holes or channels passing through the substrate are expediently each at least 100 μm2 in size and of any desired, but expedient, geometrical form. The surface area of these holes or channels is set an upper limit by the mechanical stability of the thin film acting as a membrane and of the substrate itself.
As mentioned, a major advantage of thin films in the submicron range is that the grains exhibit only limited growth over time; they no longer grow once they reach a grain size dependent on the material and the production method. This relaxation time generally lies between 5 and 20 hours, in particular around 10 hours. An essentially stable grain size can be maintained at temperatures up to preferably approximately 1100° C.
An approximately stable grain size is understood in the present case as meaning that the remaining deviation after the relaxation time is at most approximately ±5%, preferably at most approximately ±2%. In the case of a grain size of, for example, 500 nm, the subsequent grain growth lies in the range of at most approximately 25 nm, in particular at most approximately 10 nm.
The individual layers of the thin film may be of any desired thickness; it expediently lies in a range of from 5 to 10,000 nm, preferably from 10 to 1000 nm, with an average grain size K of at most approximately 200 nm, preferably from 5 to 100 nm. With respect to the layer thickness of an individual layer of the thin film, the grain size K is preferably at most approximately 50%, in particular at most approximately 20%. Here and hereafter, an amorphous layer structure is not specifically mentioned but is analogously attributed to the fine-grained thin films.
The thin film preferably always has at least one layer that is ionically or ionically and electronically conducting, in particular for O2 ions. This layer, the solid electrolyte, is always predominantly ionically conducting, and at most slightly electronically conducting.
The overall electrical conductivity is generally in the range from 0.02 to 105 S/m (Siemens/meter). Electrical conductivity may be required on an application-related basis, for example in the case of electronically active electrodes and electrolytes that are used together or individually as miniaturized sensors, gas separating membranes or solid oxide fuel cells (SOFC).
The thin film may comprise various layers of a laminar structure that are in themselves homogeneous, with a chemical composition, morphology and/or porosity that is slightly changed continuously from layer to layer, a gradient being established with respect to the chemical composition, morphology and porosity. If, for example, one or more layers of the thin film is or are porous, the porosity is in a range from >0 to 70% by volume. The porosity may vary from layer to layer, with a continuous increase or decrease to form a porosity gradient.
The thin film that is used most frequently in practice comprises an anode layer, a solid electrolyte layer and a cathode layer, all the layers being electrically conducting. Depending on requirements, these layers may comprise further layers lying in between or formed as outer layers.
The layers of the thin film consist of at least one oxide ceramic and at least one metal, but also of a mixture of at least one oxide ceramic and at least one metal; the latter composition is also known as cermet. A thin film may not be purely metallic; at least one layer must be predominantly ionically conducting. The layers of the thin film may be amorphous, two-phase amorphous-crystalline or completely crystalline.
A fluid distributor for the fluid flowing through or flowing over is arranged in the free surface of the substrate, that is to say in the side of the surface that is facing away from the thin film, and is expediently formed in the shape of a channel or groove. The channel-shaped embodiment passes through the substrate completely; it is covered by the membrane stretched over. A groove-shaped fluid distributor, on the other hand, is merely cut out partially from the substrate in terms of depth.
It is particularly advantageous to provide between the layers forming the SOFC and the substrate a metal layer (as the lowermost layer of the thin film), which acts in the exposed parts of the membrane as a protective layer, etching resist, bonding layer, seal or diffusion barrier between the substrate and the layers that follow. On account of its metallic conductivity, the lowermost layer also serves for the electrical contacting of the layers lying above. Depending on the function, the metal layer may be structured during application or subsequently, or be impermeable or porous. Porosity or a holey structure are advantageous if gas access to the thin film lying above is to be ensured. The metal layer consists with preference of at least one of the substances Co, Fe, Cr, Ti, Cu, Au, Ag, Ni, Pt, Ta, Si, Pd, Ru or Rh. If required, an insulating layer consisting of SiCx, SiNx and SiOx may be applied. According to a further special embodiment of the invention, at least part of the composite region between the thin film and the glass substrate may comprise a heating element, which can additionally introduce heat, for example into a miniaturized SOFC.
A thin film applied to the substrate preferably has a grain size of at most approximately 500 nm in all the layers. In at least one of these layers, an essentially stable grain size is retained after a relaxation time, even in an elevated temperature range, which has positive effects on the properties.
The individual layers of the thin film covering the openings in the substrate do not have to be of the same size with respect to surface area. At least one layer of the thin film must cover at least one of the substrate openings. Each of the other layers of the thin film may cover this first layer entirely or partially or extend beyond the first layer. The layers of the thin film acting as a membrane may be structured by selective depositing or etching, by lift-off or masking techniques, or by any desired combination of these forms of deposition or in any desired form.
For miniaturized devices with electrochemically active electrodes and a solid electrolyte, a thin film with at least three of these fine-grained layers one on top of the other may be applied to a substrate as a membrane. As mentioned at the beginning, the working techniques are known per se.
One or more layers of the thin film may consist of a metal or a metal oxide, for example of Cu, Co, Mn, Ag or NiOx, FeOx, MnOx, CuOx, CoOx, and MnOx, AgOx, RuOx or mixtures of metals and or metal oxides. Furthermore, an oxide-ceramic component with ionic or mixed ionic and electronic conductivity, such as for example doped ceroxide AxCe1−xO2−δ, where A=Gd, Sm, Y, Ca, 0.05≦x≦0.3, or doped zirconium oxide LnyZr1−yO2−δ, where Ln=Y, Sc, Yb, Er, 0.08≦y≦0.12, may be added to the metal, metal oxide or the mixture of metal and metal oxide. The proportion by volume of the metal and oxide-ceramic component lies between 20 and 80% by volume. The proportion by volume of the metallic phase of the solid part of the cermet lies between 0 and 70% by volume. The ratio between metal and oxide ceramic may be both uniformly distributed and singly or multiply graduated over the film thickness, with a ratio between 0 (no metal in the layer) and 100% (pure metal layer) of metal at each location of the thin film. The porosity of the thin film ranges from 0 to 50% in the oxidized state; all the metallic components are in the form of metal oxide, and >0 to 70% for the reduced state; all the metallic components are in the form of metal with a homogeneous or a non-homogeneous distribution in the thin film. The porosity may take the form of a gradient from impermeable to 70% porosity of the thin film. The average grain size K of the materials can be determined by thermal annealing at different temperatures; it comprises average grain sizes K of from 5 to 500 nm. The oxide-ceramic phase of the layers of the thin film have stable microstructures as a function of time under reducing conditions at temperatures of up to 700° C. If the metal content lies above a certain limit volume from which the metallic conduction becomes perceptible, the overall electrical conductivity between room temperature and 700° C. is greater than 10 S/m; the metal is in a reduced, that is to say metallic, state. All these materials can be coated, impregnated or doped with the following metals, or form composite materials with these metals, for example Ag, Au, Cu, Pd, Pt, Rh and Ru.
Furthermore, one or more of the layers of the thin film may consist of doped ceroxide AxCe1−xO2−δ, where A=Gd, Sm, Y, Ca, 0.05≦x≦0.3, or of doped zirconium oxide LnyZr1−yO2−δ, where Ln=Y, Sc, Yb, Er, 0.08≦y≦0.12, or of LaxSrxGa1−yMgyO3±δ, with 0≦x≦1 and 0≦y≦1. The layers of this thin film are of an impermeable nanostructure and have a film thickness of between 10 and 5000 nm. A thin film with layers of an average grain size K of between 5 and 500 nm can be produced. This thin film has the following electrical properties:
a) An overall electrical conductivity of between 0.02 and 5 S/m at 500° C. and 0.25 and 10 S/m at 700° C., both measured in air.
b) An activation energy of the electrical conductivity in air of between 0.5 and 1.5 eV within the temperature range of 100 to 1000° C.
c) The electrolytic domain boundary is at 500° C. under oxygen partial pressures lower than 10−19 atm and at 700° C. under oxygen partial pressures lower than 10−14 atm.
Furthermore, one or more layers of the thin film may consist of a perovskite of the type AxA′1−xByB′1−yO3±δ, where A, A′, B and B′ are any of the following elements: Al, Ba, Ca, Ce, Co, Cu, Dy, Fe, Gd, La, Mn, Nd, Pr, Sm, Sr, Y and 0≦x≦1, 0≦y≦1. According to a subvariant, pyrochlore ruthenates of the composition A2Ru2O7±δ, where A=Bi, Y, Pb or A2−αA′αMO4±δ with (A=Pr, Sm; A′=Sr; M=Mn, Ni; 0≦α≦1) or a material of the following composition: A2NiO4±δ (A=Nd, La); AxByNiO4±δ with A, B=Al, Ba, Ca, Ce, Co, Cu, Dy, Fe, Gd, La, Mn, Nd, Pr, Sm, Sr, Ya and 0≦x≦1, 0≦y≦1, or La4Ni3−xCoxO10±δ, or YBa(Co,Fe)4O7±δ or Baln1−xCoxO3±δ or Bi2−xYxO3 (0≦x≦1) or La2Ni1−xCuxO4±δ (0≦x≦1), or Y1Ba2Cu3O7 is used. All these materials can be coated, impregnated or doped with the following metals or form composite materials with these metals: Ag, Au, Cu, Pd, Pt, Rh and Ru. Furthermore, the thin films may comprise a mixture of these materials with doped ceroxide AxCe1−xO2−δ, where A=Gd, Sm, Y, Ca, 0.05≦x≦0.3, or doped zirconium oxide LnyZr1−yO2−δ, where Ln=Y, Sc, Yb, Er, 0.08≦y≦0.12, or La1−xSrxGa1−yMgyO3±δ, where 0≦x≦1 and 0≦y≦1. The thin films preferably have a layer thickness of between 50 and 10,000 nm and an average grain size K of between 5 and 500 nm. The overall electrical conductivity at 550° C. is in the range between 10 and 100,000 S/m in air. The thin films are stable in air and may be impermeable or porous with a porosity of between >0 and 70% by volume.
Finally, in addition to at least one ceramic or cermet layer, one or more layers of the thin film may be in the form of a metal or a metal mixture, for example Pt, Au, Ag, Ni and others, which are produced by sputtering techniques, such as RF (radio frequency) or direct-current sputtering, a vapor depositing technique or any other vacuum technique, electrochemical deposition or a paste of metal oxide powder and any organic or non-organic component.
With respect to the method, the object is achieved according to the invention by regions being selectively dissolved out of the structurable glass substrate.
Preferably, the parts of the surface that are to be removed are exposed to UV, heated up for at least partial, selective transformation into glass ceramic, and the exposed parts are specifically removed. The parts that are not exposed may be subsequently ceramicized.
The method is carried out by means of process techniques that are known per se. The untreated glass substrate is covered with a mask or a photoresist in such a way that only the parts of the substrate that are to be removed are left free, while the other parts are covered. Exposure to UV rays and subsequent heat treatment in an oven are performed in accordance with specifications of the glass manufacturer. The exposed parts of the glass substrate are thereby crystallized at least partially. After the removal of the substrate from the oven, the at least partially crystallized glass ceramic is dissolved out, for example by etching, likewise in accordance with the specifications of the glass manufacturer. This produces exactly delimited holes or channels, which reach as far as the protective layer mentioned or as far as the thin film, which are not attacked, or only a little, by the etchant. Finally, the remaining substrate can be completely or further ceramicized in an oven. As mentioned, we are concerned here with miniaturized substrates; the holes are at least 100 μm2 in size, that is to say in the case of square openings have a side length of at least 10 μm. The surface area of a clearance generally remains well below one mm2.
The main use of a composite element according to the invention comprises use in a miniaturized electrochemical device, in particular in a solid oxide fuel cell, a sensor or as a gas separating membrane.
The invention is explained in more detail on the basis of exemplary embodiments that are represented in the drawing and are the subject of dependent patent claims. In the schematic cross sections:
Represented in
A fraction of a gas inflow impinging on a gas-impermeable glass substrate 12 according to
The thin-film membrane 10 with the electrochemically active layers of the miniaturized fuel cell 18 essentially comprises
an anodic first layer S1 of a cermet, resting on a rigid glass substrate plate 12 with holes 14 or channels 15,
a second layer S2, also laterally covering the anode and formed as a solid electrolyte, and
a cathodic third layer S3, resting on the solid electrolyte.
The anodic layer S1 and the cathodic layer S3 are each connected to a metallic current conductor 20, 22 and lead the direct electric current that is generated via a load 24. The electrodes S1, S3 may contain catalytically active metal particles.
The electrode layers S1 and S3 are formed such that they are gas-permeable; the electrode layer S2 is gas-impermeable, but permeable to oxygen ions, which is indicated in
As shown in
Represented in
A miniaturized SOFC 18 that is represented in
A further variant according to
In
The solid electrolyte that is permeable to oxygen ions, layer S2, consists in the present case of ZrO2 doped with 8% Y2O3. The resistance measured over current conductors 20, 22 is fed to a measuring instrument 38 with a display area.
In
In the next method step b, the substrate 12 is exposed to UV and then, in a further method step c, after removal of the photoresist 42, is heat-treated in an oven. The parts 44 of the glass substrate 12 that are exposed to UV rays are thereby partially ceramicized.
The next method step comprises etching out the partially ceramicized parts 44 of the glass substrate 12 as far as the protective layer S0,B (
In a final method step d, the parts 40 of the glass substrate that are not exposed to UV rays are put into an oven along with the thin-film membrane 10 and completely ceramicized.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1682/05 | Oct 2005 | CH | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CH2006/000575 | 10/17/2006 | WO | 00 | 9/20/2008 |
