Coating processes – Electrical product produced – Metal coating
Reexamination Certificate
1999-07-23
2001-10-23
Warden, Sr., Robert J. (Department: 1744)
Coating processes
Electrical product produced
Metal coating
C427S304000, C427S305000, C427S430100, C204S429000, C205S170000, C205S183000
Reexamination Certificate
active
06306457
ABSTRACT:
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 NO
x
-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.
REFERENCES:
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patent: 4199425 (1980-04-01), Sinkevitch
patent: 4541905 (1985-09-01), Kuwana et al.
patent: 4863583 (1989-09-01), Kurachi et al.
patent: 5006221 (1991-04-01), Uchikawa et al.
patent: 5080689 (1992-01-01), Pal et al.
patent: 5326597 (1994-07-01), Sawada et al.
patent: 2198750 (1988-06-01), None
Schneider Jens Stefan
Schumann Bernd
Stanglmeier Frank
Kenyon & Kenyon
Olsen Kaj K.
Robert & Bosch GmbH
Warden, Sr. Robert J.
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