Coating system and method for its manufacture and its use

A coating system and a method for its manufacture are provided. An electrically conductive base coat and a porous overcoat lying over the base coat are arranged on a ceramic substrate. At least one additional deposited layer is arranged on the base coat in such a way that the additional layer is formed in the pores of the porous overcoat adjacent to the base coat. The additional layer is deposited either by currentless or electrolytic deposition. For electrolytic deposition of the additional layer, the ceramic substrate sintered with the base coat and the overcoat is submerged in an electrolytic bath and the base coat is connected as a cathode. The currentless deposition takes place from a solution of the metal to be deposited with the addition of a reducing agent.

FIELD OF THE INVENTION

The present invention relates to a coating system, and a method for the manufacture and use of the coating system

BACKGROUND INFORMATION

Conventional coating systems can be found, for example, in electrochemical oxygen sensors in which a ceramic body produced from a solid electrolyte is provided with at least one electrode exposed to a gas to be analyzed, and a porous overcoat covering the electrode. The electrode is made up of a catalytically active material such as platinum which is capable of adjusting the equilibrium setting of the gas to be analyzed on the electrode surface.

U.S. Pat. No. 4,199,425 describes a sensor in which an additional catalytic material, rhodium, is introduced into the pores of the porous overcoat by impregnation and subsequent calcination. The rhodium precipitates onto the pore walls of the entire overcoat in the form of ultra-fine particles so that no specific coating thickness can be set in the porous overcoat.

A method for the currentless deposition of metals onto metallic surfaces and the monitoring of these processes is described in British Patent No. 2 198 750. However, this method does not make the specific application of a metallic coating onto an electrode surface through a porous protective coating possible.

SUMMARY

An advantage of the coating system according to the present invention is that one or more additional layers having a defined layer thickness are formed on an electrically conductive base coat. Another advantage is that the additional layer or layers arranged immediately adjacent to the electrically conductive base coat does not or do not completely fill up the pores of the porous overcoat. This preserves the protective effect of the porous overcoat as well as an adequate gas transfer through the overcoat. The method according to the present invention makes it possible to deposit the additional layers onto the base coat through the porous overcoat after the ceramic body has already been sintered. As a result, materials can be used for the additional layers that otherwise would not stand up to the high sintering temperature.

The subsequent electrolytic or currentless deposition of at least one layer on the base coat makes it possible to modify the functional properties of the base coat. This is particularly advantageous for the modification of the functional properties of an electrode in gas sensors with regard to their specific gas selectivity and/or control layer.

A particularly marked influence of the materials of the base coat and the additional layer on each other is achieved by a thermal aftertreatment of the coating system after the additional layer has been deposited. For example, a temperature range of 1200° C.±100° C. has proven to be favorable for an Au/Pt coating system. At this temperature, the metal atoms of the additional layer diffuse into the metal of the adjacent base coat. Such a mixing phase of the materials is necessary, for example, for electrodes of gas sensors intended to respond to a specific gas species. For example, in order to form an HC-selective or NOx-selective sensor, the electrode of a gas sensor can be modified in such a way that the electrode then has a special affinity for hydrocarbons or nitrogen oxides. It is further possible to adjust the catalytic properties and the thermal properties of the gas sensor by the selection of the material for the additional layer. Moreover, the control layer of the sensor can be influenced by the selection of the material and/or the thickness of the deposited layer.

An advantage of an currentless deposition of an additional layer onto a base coat in relation to electrolytic deposition is that only electrically contacted compartments of the base coat are coated in electrolytic deposition whereas all the particles on the surface of the base coat are coated in currentless deposition. This is advantageous since parts of the base coat that are electrically insulated at room temperature can definitely be contacted at the very high operating temperatures of a gas sensor via the solid electrolyte substrate which is then conductive. Thus, when the coating system is used as a measuring electrode and these parts are not coated, they have an unfavorable influence on the resulting sensor signal.

A further advantage is that a cermet layer is used as the electrically conductive base coat, the cermet layer forming a solid connection with the ceramic substrate during sintering of the ceramic body due to the ceramic component of the cermet layer.

DETAILED DESCRIPTION

The coating system of the present invention has, for example, the layer structure shown inFIGS. 1or2. According to the coating system inFIG. 1, an electrically conductive base coat13made up of a Pt-cermet and having an electrical terminal contact35is arranged on a ceramic substrate11made up of a solid electrolyte such as ZrO2. A porous overcoat15is arranged on base coat13. Adjacent to base coat13, an additional layer21is formed on the base coat in the pores of overcoat15. Layer21is thus in direct contact with base coat13.

FIG. 2shows a second exemplary embodiment of a coating system. In this case, layer21is formed in the pores of overcoat15over base coat13and a second layer22is formed over layer21and a third layer23is formed over layer22. Layer21is of gold, layer22of rhodium or iridium and layer23is of nickel or chromium. This embodiment shows that even a complex, multilayer coating structure can be implemented in a simple manner. As a mixed potential electrode, such a coating system is used in mixed potential sensors. Mixed potential electrodes are electrodes which are not able or not completely able to catalyze the equilibrium setting of a gas mixture on their surface. If a mixed potential electrode is connected together with a reference electrode of platinum, such an arrangement then forms a mixed potential sensor. An appropriate selection of material for additional layer21makes it possible to set the selectivity of the resulting electrode specifically for one gas species and/or to specifically modify the control layer of the sensor. Thus, for example, the low temperature characteristics of an oxygen sensor can be improved by a rhodium layer on a Pt electrode. With a layer structure shown in FIG.2and via an appropriate selection of materials for layers21,22,23, it is also possible to specifically modify the catalytic properties of the electrode surface in addition to setting the selectivity.

To manufacture the coating system according toFIG. 1, ceramic substrate11provided with electrically conductive base coat13and porous overcoat15is sintered at a temperature of 1400° C. However, it is also possible to apply overcoat15to base coat13only after sintering. Not only ZrO2, but also Al2O3is suitable as ceramic substrate11.

In the present exemplary embodiments, ceramic substrate11is provided with a layer21according to FIG.1and with more than one layer21,22,23according toFIG. 2, layer21or layers21,22,23being formed in the pores of porous overcoat15in superimposed strata. Two examples of how the layers21,22,23can be formed are illustrated inFIGS. 3 and 4.

A first example is to produce additional layers21,22,23by electrolytic deposition. A structure based on this method is shown in FIG.3.

For this purpose, ceramic substrate11is placed into an electrolytic bath31; base coat13is electrically contacted at terminal contact35and connected as cathode37. An electrode made of a metal corresponding to the metal of the particular layer21,22,23to be deposited is used as anode33(electrolytic process with sacrificial anode). Water-soluble salts of the metals in question, such as HAuCl4, IrCl3x H2O or RhCl3x H2O, serve as the electrolyte.

In order to manufacture a sensor to detect hydrocarbons, a coating system according toFIG. 1is selected, a gold layer being electrolytically deposited as additional layer21onto base coat13of Pt-cermet. For this purpose, the sintered ceramic body of the sensor is placed into electrolytic bath31with an HAuCl4electrolyte and a gold anode is used as anode33. At a current intensity of 0.5 to 2 mA and a current duration of 15 to 50 minutes, layer21of gold is deposited onto the Pt-cermet base coat13at a layer thickness of 1-5 μm. Layer21is formed in the pores of overcoat15. After deposition of layer21, the ceramic body is subjected to a tempering at a temperature of 1200° C. During the tempering, an alloy forms between the platinum of base coat13and the gold of layer21, the alloy being namely a platinum-rich gold phase and a gold-rich platinum phase. As a result, the catalytic activity of the platinum of base coat13is modified and a mixed potential electrode is formed.

Depending on the area of application, electrolytically produced layer21may be made from a noble metal (such as gold, rhodium, iridium), a semi-noble metal (such as palladium, silver), a base metal (such as copper, bismuth, nickel, chromium) or a mixture of these metals.

A coating system according toFIG. 2may also be produced electrolytically, the corresponding anode materials and/or the corresponding electrolytic baths being used successively in the electrolytic deposition.

Additional layers21,22,23may also be produced by currentless deposition. An apparatus based on this method is shown in FIG.4. For this purpose, ceramic substrate11with base coat13and porous protective coating15is submerged in a metallic salt solution or in a solution of a suitable metal complex32of the metal to be deposited. After the addition of a chemical reducing agent39via a metering device38, the corresponding metal is deposited with a time delay depending on the nature of the solution. In the process, the added reducing agent produces nascent hydrogen in a first step on the surface of metallic base coat13, the nascent hydrogen for its part being capable of reducing the metallic salts or metal complexes contained in the solution to elementary metal which then precipitates. An advantage of a direct participation of the electrode surface in the deposition process can primarily be seen in the fact that the metal precipitates in direct contact with base coat13and not in the pores of the entire porous protective coating15.

In order to manufacture a mixed potential sensor, a coating system according toFIG. 1is used, an additional layer21of gold being deposited by currentless deposition on base coat13made from a platinum cermet. For this purpose, a ceramic substrate of ZrO2, to which base coat13of a platinum cermet is applied and which is covered by a porous protective coating15, is submerged in a solution32of 5 g HAuCl4in 250 ml water and 50 ml of a 37% formaldehyde solution is added via metering device38. The solution is heated to 60 to 80° C. with the aid of a heating unit (not shown). The progress of the gold deposition can be readily followed via the discoloration of metallic salt solution32. After deposition is completed, ceramic substrate11is removed from the metallic salt solution and a rinsing and drying treatment takes place. If the coating system is subsequently tempered at a temperature of 1200° C., an alloy is formed between the platinum of base coat13and the deposited gold of layer21. Owing to the lack of catalytic activity, the resulting coating system is suitable as a mixed potential electrode of a mixed potential sensor.

Au, Ni, Co, Cu, Ag, Sn or W may be used as additional metals that are particularly suited for currentless deposition. Primarily aldehydes such as formaldehyde, hydrazine and alcohols are suitable as reducing agent39.

In order to achieve a complete penetration of porous protective coating15with the corresponding metallic salt solution or metal complex solution as rapidly as possible, a vacuum may be applied to the deposition apparatus during deposition or the apparatus may be subjected to ultrasound treatment.

The deposition rate is controlled primarily via the temperature and the pH of the solution. The deposition process is followed by a rinsing and/or drying process. The resulting coating system may, as already described, be subjected to a heat treatment.

The present invention is not limited to the described exemplary embodiments, but rather additional combinations and coating systems beyond the coating systems shown inFIGS. 1 and 2and described are possible in which a metallic layer in a porous layer are deposited on an electrically conductive and/or metallic base coat.