DE2539001A1 - METHOD OF APPLYING ELECTRODES TO CERAMIC ELECTROCHEMICAL GAS ANALYZERS - Google Patents

Description

Patent attorneys 0 00 Munich 2 2 Steinsdorfstrasse 21 - 22 Telephone 089/29 84 62

B 7528

THE BROKEN HILL PROPRIETARY COMPANY LIMITED 140 William Street, Melbourne, Victoria, Australia

Method for applying electrodes to ceramic electrochemical gas analyzers

The invention relates to a method for applying electrodes from a suitable noble metal such as platinum, palladium, gold or silver on a solid ceramic electrolyte body, for example made of Zirconium oxide is used to manufacture or repair sensors that are used in the analysis of gases such as oxygen can be. The invention also relates to a sensor which is produced according to this method.

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Gas sensors of the type described above are widely used in the Measurement of the oxygen content in gases, for example to monitor the efficiency of combustion processes or for suitability of a gas when used for purposes other than an inert gas atmosphere. Its main area of application is iron and steel industry, although the sensors in question are also used in other technical areas, such as brick ovens Incinerator, automobile exhaust and the like are used will.

Platinum electrodes for electrochemical oxygen gas analyzers with stabilized As is known, zirconium oxide electrolytes are produced by applying a platinum paste. The platinum paste contains a suspension of finely divided Platinum in an organic binder. The platinum paste is applied to the zirconium oxide electrolyte, air dried and then in fired in an oxidizing atmosphere at about 1,000 ° C, whereby one a sintered porous platinum electrode is obtained. When used, these electrodes tend to peel off or crumble off the zirconium oxide substrate. The reasons for this are as follows:

1. Reducing gases cause the platinum to recrystallize subsequent brittleness.

2. Phase changes, such as boiling water, in porous Structure.

3. Combustion of unburned reducing gases with excess oxygen in a porous structure.

4. Reaction of fine dust particles from fly ash, slag and the like. with zirconium oxide after it has been introduced into the porous platinum, which results in the zirconium oxide breaking to form the platinum bond.

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5. An evaporation of platinum when the ambient temperature is 1,000 C exceeds.

The object of the invention is therefore to provide an electrochemical gas sensor and a method for producing this sensor, in particular a method for applying the electrodes to a solid electrolyte substrate to show.

The invention provides an electrochemical one to solve this problem Gas sensor before, consisting of a solid electrolyte substrate a thin substantially continuous layer of an electrode metal which is connected to the substrate and a relatively thicker electrically conductive metal, which is connected to the thin metal layer.

In a preferred embodiment of the invention, this contains relative thicker metal a layer of a porous electrode metal, which with the thin metal layer is connected.

Another preferred embodiment provides that the relative thicker metal has a thin electrically conductive metal wire, which is connected to the thin metal layer or on the thin Metal layer adheres.

An electrolyte made of zirconium oxide is preferred as the electrode metal bare platinum is preferred. If a porous electrode is desired, the electrode is made of calcined platinum paste. if If a wire is used as the electrode, this wire is also preferably made of platinum.

The bare platinum can, for example, be approximately 0.08 μm to 1.5 μm thick and has a preferred thickness of approximately 1 μm. The porous electrode can be preferably from about 5 to about 500 μηι μΐη be thick, although thicker electrodes in extremely hot environments (about 1300 ⁰ C),

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where there is an excessive loss of platinum can. The platinum wire can have a diameter of about 0.05 mm to 0.125 mm.

The lower limit of the thickness of the thin layer is quite arbitrary and is determined by the need for a continuous layer, for which a good electrical conductivity can serve as a display parameter »while the upper limit is due to the need for a good one mechanical adhesion to the electrolyte is determined.

In practice, the sensor can preferably be a zirconium oxide tube have which has a closed end. The electrode can be formed according to one of the embodiments described above and is located at least on the outside of the tube. Of course, the invention can also be used for sensors with a different geometric configuration come into use.

The invention also provides a method in which an electrode on a solid electrolyte substrate or on a solid electrolyte base is applied. Here, an essentially coherent thin coating of an electrode metal is applied, dried, and the coating is then baked so that the coating adheres or bonds to the substrate. With the thin one A thicker electrically conductive metal is bonded to the coating as compared to the coating.

In a preferred embodiment of the thicker metal, a Layer of electrode metal paste used and applied to the fired thin coating. The metal paste layer is fired so that it forms a porous electrode layer with the thin coating connected is.

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Another embodiment is that the thicker metal than thin metal wire is formed and is connected to the thin coating by another thin coating of a similar or the same Electrode metal is applied over the wire.

The thin layer is preferably made of bare platinum, which is applied in the aforementioned thick area. The firing temperature can be selected from 65O ⁰ C to 800 ° C, the firing being for instance of air or oxygen, carried out in a highly oxidizing atmosphere. The annealing step includes heating the layer from room temperature to a temperature in the aforesaid range for a time of about two to three hours, followed by annealing at the annealing temperature for not less than about 20 minutes.

A platinum paste with a thickness of 5 μπα to 500 μια is preferably selected as the thicker porous layer. A preferred layer thickness is 100 μm, which is used at lower temperatures <1,000 C. A layer thickness of 500 μm is used for high temperatures. The annealing and firing the paste layer contains a heating up to about 800 to 1,000 ⁰ C for a period of about 3 or 4 hours followed by annealing for not less than 20 minutes at the baking and annealing temperature.

A preferred embodiment of the method is described below, that has been successfully verified:

1. An intermediate layer of an organometallic platinum compound (bare or light platinum "bright platinum") is applied to the zirconium oxide by painting. The layer is dried in the air and annealed or fired in a stream of oxygen or air at 700 ^° C. for 20 minutes. The temperature is increased slowly over three hours to avoid the formation of bubbles. It was found,

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that a continuous layer resulted, which was strongly adhered. In order to ensure a continuous layer with certainty, if necessary, a second coating can be carried out.

2. On the bright or bright platinum layer is, for example, by stress or on a layer of platinum paste by dipping, which is dried in air, and is brought slowly for four hours at 850 ⁰ C under oxidizing conditions. The layer is held at this temperature for about an hour. The final electrode thickness was about 100 μm. However, the layer thickness should not exceed about 500 μm, since there is a risk that a thick layer will peel off. The pre-layer of bare or light platinum acts as a strong adhesive on the substrate to which the platinum paste layer is bonded. The upper layer of the platinum paste ensures the majority of the electrical conductivity of the electrode combination, while the bare or bright platinum layer improves the cell resistance, so that the use of a cheaper display device with low resistance is made possible.

Since the bare or light platinum layer is continuous, it protects that Zirconium oxide against dust reactions, but is thin enough so that the penetration of oxygen or reducing gases is not prevented, while at the same time providing strong adhesion to the substrate.

Preferably, the electrode is made so that on both Side of the electrolyte an electrode is applied. One electrode serves as a reference electrode: and the other serves as a variable electrode.

If the reference gas is air, the reference electrode can be a porous standardized one Be a platinum electrode or porous platinum standard electrode without the bare or light platinum base layer. If zirconium oxide tubes with closed At the end of the application, they have an outer diameter

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knife of about 10 mm and an inner diameter of about 8 mm. Electrodes as described above are applied to this tube. It has been found that such electrodes lake cell resistors 25-50 Q, Glßichgewichtsausgangsspannungen 48.0 to 61.0 mV at 700 ⁰ C for a calibration gas, and response 15-180. to achieve a balance output voltage greater than 94%. It was found that the parameters were sufficiently fair to the cells.

In a preferred embodiment, the porous platinum layer is replaced by several rows or layers of thin metal wire, which covers the thin coating layer of the electrode metal. The wire is preferably covered with the base coating by a further coating of an electrode metal and is used to forward the voltage signal that has arisen on the electrode layer to a suitable measuring instrument. In each case, the electrode metal coating is applied based on the conditions that were used in the first Embodiment have been determined.

In a preferred embodiment, the base metal or base metal is used and platinum is used for the further coating, which is applied in the manner described in the first embodiment. The thicknesses used there are also used. The wire diameter can be in a range from 0.05 mm to 0.125 mm.

In one example, 0.125 ram wire was used and wound onto a closed-ended zirconia tube. This tube had an outside diameter of about 10 mm and an inside diameter of about 8 mm. 5 turns per millimeter were applied over a length of about 1 cm. This embodiment has been found to be effective in reducing gas spots, that is, the output voltage of the cell is in the range of 750 to 1,300 mV

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for a ZeUesi ^ mperatur in the range from 600 ⁰ C to 100 ° C. In addition, the cell constructed in this way has an extremely short response time of around just one second around 1,300 mV starting from 0 mV. to reach. The response time of this embodiment is significantly faster than that of the first embodiment. This is due to the significantly smaller surface area of the bright or bare platinum electrode layer and the smooth nature of the surface that is exposed to the gas to be tested.

Since the thin wire is used in this embodiment to make the The electrode metal only needs to pass on the voltage signal to the area of the tube or another fire-resistant body be applied, which is exposed to the maximum temperature of the gas to be tested. Because the tip of the tube is exposed to the maximum temperature the electrode and wire is applied to a short area near the tip. The length over which the wire will be wrapped should depend on the diameter of the wire, but can be varied between about 3 and 10 mm.

The specified range for the appropriate wire diameter is pretty freely definable. The lower limit is determined by the practical treatment characteristics of the wire, while the The upper limit is determined by the desire to keep the wire diameter as small as possible. The mentioned upper limit is selected in terms of striving to have as much contact points as possible with the To have a platinum electrode, so that a good transition or a good transmission of the voltage signal which is developed at the electrode, is guaranteed. It also plays a role that the dimensions of the first embodiment should not be exceeded, so that the second embodiment is interchangeable '? with the first embodiment. Of course, if a larger cell is chosen, a wire diameter over 0.125 mm would be an advantage.

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The preferred embodiments have been made in connection with a Described zirconium oxide electrolyte having a platinum electrode. Of course, the invention can also be used with other electrolyte-electrode combinations come into use.

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Claims (11)

Claims 1. Electrochemical gas sensor, characterized by a solid Electrolyte substrate or a solid electrolyte pad, a thin one substantially continuous electrode metal layer, which is connected to the substrate or the base and by a comparison to the first layer (intermediate layer) thicker electrically conductive metal, which is connected to the thinner electrode metal layer. 2. Electrochemical gas sensor according to claim 1, characterized in that that the thicker electrically conductive metal in the form of a thicker layer than the intermediate layer of a porous electrode metal is connected to the thinner electrode metal layer (intermediate layer). 3. Electrochemical gas sensor according to claim 2, characterized in that that the thin electrode metal layer (intermediate layer) consists of a noble metal and a thickness of about 0.08 μΐη to 1.5 ^ m and that the porous electrode metal layer is an identical or similar metal with a thickness of 5 μm to 500 μm. 4. Electrochemical gas sensor according to claim 1, characterized in that that the thicker electrically conductive metal, which is connected to the thinner metal layer (intermediate layer), than thinner electrically conductive metal wire is formed, which is connected to the thin metal layer. 5. Electrochemical gas sensor according to claim 4, characterized in that that the wire is connected to the thin intermediate layer by a similar or identical electrode metal layer, which is about the thickness of the having first layer or intermediate layer. ⁷⁵²⁸ 609812/0898 ΛΑ 6. Electrochemical gas sensor according to claim 5, characterized in that that the thin electrode metal layer (intermediate layer) consists of a noble metal and a thickness of about 0.08 μm to 1.5 μm and that the wire diameter of the wire, which is formed from the same or similar metal, is about 0.05 mm to 0.125 mm having. 7. Method of applying an electrode to a solid electrolyte substrate or on a solid electrolyte base, in particular for the production of an electrochemical gas sensor, characterized in that, that a substantially continuous thin coating of an electrode metal is applied to the substrate or on the base, that the Coating is dried and fired or annealed, so that the coating bonds with the substrate or with the base and that with this thin coating an electrically conductive metal or an electrically conductive metal body is connected, which is thicker than the thin coating is. 8. The method according to claim 7, characterized in that the thicker Metal or the thicker metal body in the form of a layer of an electrode metal paste on the annealed or fired thin coating is applied that the paste layer is annealed or fired, so that a porous electrode layer is connected to the thin coating will. 9. The method according to claim 7, characterized in that the thicker one Metal or thicker metal body a metal wire is used, which with the thin coating by applying a further thin coating from a similar or identical electrode metal connected to the wire will. $ 09812/0898 10. The method according to any one of claims 7-9, characterized in that the thin coating (intermediate layer) of the electrode metal is formed from a noble metal, in particular platinum, and the coating is heated to a temperature of about 650 800 ^{0 during drying and annealing or firing} C is heated in a strongly oxidizing atmosphere of air or oxygen for a period of about 2 or 3 hours and that annealing is carried out at that temperature for not less than about 20 minutes. 11. The method according spoke 8 or 10, characterized in that a noble metal, in particular platinum, is used for the metal paste and during firing or annealing the layer to a temperature of about 800 ⁰ C to 1,000 ⁰ C for a time of 3 to Is heated for 4 hours and then calcined at this temperature for a time of not less than about 20 minutes. 60981 2/0898