WO1997034331A1

WO1997034331A1 - Method of coating a component of a high-temperature fuel cell with a vitreous layer and high-temperature fuel cell stack

Info

Publication number: WO1997034331A1
Application number: PCT/DE1997/000382
Authority: WO
Inventors: Thomas Jansing; Ludger Blum
Original assignee: Siemens Aktiengesellschaft
Priority date: 1996-03-14
Filing date: 1997-03-03
Publication date: 1997-09-18
Also published as: EP0886886A1; CA2248916A1; JP2000506309A; AU2503197A; AU713015B2

Abstract

The invention relates to a method of coating a component (4) of a high-temperature fuel cell (2) with at least one vitreous layer (8, 10) by enamelling, thereby reducing production costs for the vitreous layer (8, 10) in relation to conventional production methods by at least a factor of 15 to 20.

Description

METHOD FOR COATING A COMPONENT OF A HIGH-TEMPERATURE FUEL CELL WITH A GLASS-LIKE LAYER AND HIGH-TEMPERATURE FUEL CELL STACK

Process for coating a component of a high-temperature fuel cell with a glass-like layer, use of the process for producing a high-temperature fuel cell stack and high-temperature fuel cell stack

The invention relates to a method for coating a component of a high-temperature fuel cell with a glass-like layer, to a use of the method for the production of a high-temperature fuel cell stack and to a high-temperature fuel cell stack.

In the case of various components, for example the planar components of a high-temperature fuel cell stack, it is necessary, for example for electrical or gas-tight insulation of the planar components from one another, to provide the planar components with a glass-like layer.

The planar components are usually connected to one another with glass solders or with composite glass solders. A suitable composite glass solder consists, for example, of a glass base made of B ₂ 0 ₃ -Si0 ₂ -CaO or B ₂ 0 ₃ -Si0 ₂ -BaO and corresponding ceramic composite parts made of, for example, Zr0 ₂ , Al ₂ 0 ₃ or MgO. One advantage of glass solders is that they can be soldered in both atmosphere and vacuum.

For example, from the "Fuel Cell Handbook" by AJ Appelby and FR Foulkes, 1989, pages 440 to 454, it is known that in a high-temperature fuel cell stack, in the literature, a fuel cell stack is also called "stack", various planar ones Components are stacked on top of each other. Under an upper composite circuit board, which has the function of an upper cover plate, the row follow a contact layer, a solid electrolyte electrode element, a further contact layer, a further composite circuit board, which is designed as a bipolar plate within the high-temperature fuel cell stack, etc. In this case, each form one between two adjacent composite circuit boards solid electrolyte electrode element, including the contact layer directly adjacent to both sides of the solid electrolyte electrode element and the side of each of the two composite printed circuit boards adjacent to the contact layer, together form a high-temperature fuel cell.

A practiced technique for joining the various planar components is with so-called green foils. In the case of such green foils, the composite glass solder is mixed together with an organic binder and, for example, by means of

Tape casting to about 200 to 400 microns thick film pulled. The binder content is usually about 20 wt.%. These green foils are then structured and applied to the desired surface of the planar component.

In such films, the total density of the green film is relatively low due to the volume fraction of the binder. After assembling the components and burning out the binder, the green film shrinks, which can be up to 80%. Since gas-carrying channels and spaces are formed between the individual planar components, which must be gas-tightly separated from one another, gas-tight separation of the various channels and spaces can be achieved only with difficulty and with great uncertainty due to the shrinkage when using green foils.

Furthermore, the binder must be removed from the green film without leaving any residue. This can lead to additional problems between the various planar components, since the substances released, such as binders, disperse yaws, solvents and plasticizers, can only reach the outside very poorly via the seal created when the planar components are joined together. A residue-free burnout of these released substances is therefore not guaranteed inside the high-temperature fuel cell stack.

The disadvantages mentioned lead to a reduction in the power density or even to the inoperative nature of the high-temperature fuel cell stack.

The invention is based on the object of specifying a method for coating a component of a high-temperature fuel cell, in which the above-mentioned technical disadvantages are largely avoided. In addition, the invention is based on the object of specifying a use of the method for producing a high-temperature fuel cell stack, in which the power density is not reduced when the components forming the high-temperature fuel cell stack are joined together. Furthermore, a high-temperature fuel cell stack for performing the method is to be specified.

The first-mentioned object is achieved according to the invention by a method for coating a component of a high-temperature fuel cell with at least one glass-like layer, the glass-like layer being produced by enamelling.

The second-mentioned object is achieved according to the invention by using the method for producing a high-temperature fuel cell stack.

The third object is achieved according to the invention by a high-temperature fuel cell stack with a plurality of components, in which the components have at least one a glass-like layer are connected to one another, the glass-like layer being produced by enamelling.

In the method for coating the building element of a high-temperature fuel cell with a glass-like layer, the glass-like layer is produced by enamelling. As a result, the disadvantages known from the prior art are largely avoided. The product of the enamel is a glass-like, chemically quite resistant layer. The enamel can be defined as melt mixtures of silicates, borates and fluorides of the glass-forming elements which solidify at a relatively low temperature, in particular Na, K, Pb and Al, which solidify through the separation of solid or gaseous substances, such as gas bubbles. The enamelling can be done either with wet enamel or with powder enamel as an enamel base. The basic material with which the enamelling is carried out is called the enamel basic material.

The technical disadvantages mentioned above are largely avoided by the use of spray processes for producing the glass-like layer. For example, atmospheric plasma spraying, high-speed flame spraying or vacuum plasma spraying can be used as the spraying method.

However, the effect that, depending on the composition of the glass-like layer which is to be applied to the component by one of these thermal spraying processes, has the disadvantage that the formation of bubbles in the glass-like layer increases after application to the component. The resulting increased porosity in the glass-like layer adversely affects the subsequent soldering process when assembling the components. As a result, the performance is reduced when the high-temperature fuel cell stack is later operated. In addition, the process-related high cost factor proves to be disadvantageous when carrying out these thermal spraying processes. All thermal spraying processes require a great deal of technical and apparatus expenditure.

By using the enamelling process, the formation of bubbles in the glass-like layer is almost completely prevented. This method for producing a glass-like layer, in which the enamel base material is preferably applied to the component by wet powder spraying, is moreover at least 15 to 20 times cheaper than the thermal spraying methods known from the prior art.

Further advantageous refinements are given in the subclaims.

To further explain the invention, reference is made to the exemplary embodiments of the drawing. Show it:

1 shows a coated component of a high-temperature fuel cell in a detail and in a schematic representation;

2 shows a section of a high-temperature fuel cell stack in a schematic representation.

According to FIG. 1, a plurality of layers 6, 8, 10, 8, 10, 12 are applied to a planar component 4 of a high-temperature fuel cell 2. The planar component 4 is, for example, a composite printed circuit board. It preferably consists of the metal CrFe ₅ Y ₂ 0 ₃ l, Haynes Alloy 230, Inconel 600 or a common industrial steel. An electrode (not shown in more detail) is supplied with an operating medium via this planar component 4 and, moreover, the Electricity is discharged from the high-temperature fuel cell 2 for use.

The ceramic layer 6 is first arranged on the planar component 4 of the high-temperature fuel cell 2. The ceramic layer 6 consists for example of Al ₂ 0 ₃ , spinel or Zr0 ₂ . In practice, however, several ceramic layers 6 of different compositions are arranged one above the other, which is not shown in this embodiment. The ceramic layer 6, whose thermal expansion coefficient is adapted to the thermal expansion coefficient of the component 4 of the high-temperature fuel cell 2, ensures good adhesion of the subsequently arranged glass-like layers 8, 10, 12 to the component 4. Electrical insulation is also achieved through the ceramic layer 6.

On the ceramic layer 6, the glass-like layer 8 is produced by enamelling, for example from one

Glass solder or a glass ceramic composite solder. In this case, glass solder is a lightly melting soldering glass which is characterized by low viscosity and small surface tensions and whose melting temperature lies in the temperature range between 600 and 1000 ° C.

Glass ceramics, on the other hand, is the name for a polycrystalline solid, which is controlled devitrification of a glass is produced. These glassy substances can be in the powdery state, i.e. in other words as an enamel base material, apply very tightly and firmly to the component.

The enamel base material for the enamelling is applied to the component 4 by wet powder spraying or by a screen printing process before the actual enamelling process. The enamel base material contains a binder, the proportion of the binder being between 5 and 20% by weight of the enamel base material. The costs saved by these cold processes compared to the thermal spraying processes are a factor of at least 15 to 20. The total cost saving in the present process is thus already achieved when the enamel base material is applied.

After applying the enamel base material, it is dried out in a defined manner. This happens for example at a temperature T between 50 and 100 ° C in a period of 2 hours to 3 days.

The enamel base material adheres poorly to the planar component 4 due to its powdery structure. The enamelling achieves a material connection between the glass-like layer 8, consisting of the enamel base material processed by enamelling, and the planar component 4. The enamelling, in other words the production of the glass-like, solid, binder-free and low-pore layer 8, takes place at a predetermined temperature T _E of, for example, approximately 800 ° C. This is above the softening temperature for the components of the enamel base material and below the soldering temperature for joining the components 4 of the high-temperature fuel cell 2. The heating rate is dependent on the components of the enamel base material and is, for example, 5K / min.

In this exemplary embodiment, a further glass-like layer 10 is arranged on the glass-like layer 8. The glass-like layer 10 can have the same or a different composition as the glass-like layer 8. A succession of different glass-like layers 8, 10 has the advantage that different requirements, such as electrical insulation, thermal linear expansion, tion coefficient and gas-tightness, can be fulfilled separately by different glass-like layers 8, 10.

The thickness of the individual glass-like layers 8, 10 is limited by the occurrence of mechanical stresses. Depending on the desired total layer thickness, the sequence of glass-like layers 8, 10 is repeated periodically. Total layer thicknesses of, for example, 500 μm can be achieved without the resulting residual stress of the arrangement of several layers 8, 10 increasing. This leads to a substantial mechanical stabilization of the entire high-temperature fuel cell.

In order to ensure good mechanical adhesion of the uppermost glass-like layers 8, 10 to a further component, which is not shown in this exemplary embodiment, a further glass-like layer 12 is applied to the glass-like layer 10 last produced by the enamelling process that no enamelling is carried out. This embodiment of the glass-like layer 12 is optional.

The use of this method is particularly suitable for the production of a high-temperature fuel cell stack.

According to FIG. 2, the section of a high-temperature fuel cell stack 20 comprises two components 22 and 24, which are connected to one another in a gas-tight manner by a glass-like layer 26 and are electrically insulated from one another, the glass-like layer 26 being produced by enamelling.

Composite glass solders are particularly suitable as glass-like substances, for example 75% by weight from a glass solder, for example B ₂ 0 ₃ -Si0 ₂ -CaO and / or BaO, and 25% by weight. consist of a ceramic, such as Zr0 ₂ , MgO or Al ₂ 0 ₃ , as a mixture.

The method described in connection with FIG. 1 for coating the component 4 of the high-temperature fuel cell 2 is therefore very suitable for connecting the components 22, 24 of a high-temperature fuel cell stack 20.

Advantageously, the glass-like layer 26 can be replaced by an arrangement consisting of the sequence of layers 6, 8, 10, 8, 10, 12, which is shown in the exemplary embodiment in FIG.

Claims

claims

1. A method for coating a component (4) of a high-temperature fuel cell (2) with at least one glass-like layer (8, 10), the glass-like layer (8, 10) being produced by enamelling.

2. The method according to claim 1, in which an enamel base material is applied by a screen printing process before the enamelling.

3. The method according to claim 1, in which the enamel base material is applied by wet powder spraying before the enamelling.

4. The method according to any one of the preceding claims, in which a plurality of glass-like layers (8, 10) are applied in succession.

5. The method according to claim 4, wherein the sequence of the glass-like layers (8, 10) is periodic.

6. The method according to any one of the preceding claims, in which a ceramic layer (6) is arranged on the component (4) before producing the glass-like layers (8, 10).

7. The method as claimed in one of the preceding claims, in which a further glass-like layer (12) which is not produced by enamelling is arranged on the glass-like layers (8, 10) last produced by enamelling.

8. Use of the method according to one of the preceding claims for the production of a high-temperature fuel cell stack (20).

9. High-temperature fuel cell stack (20) with several components (22, 24), in which the components (22, 24) through at least one glass-like layer (26) are connected to one another, the glass-like layer (26) being produced by enamelling.