Sealing element for a sensor

Details Sealing element for a sensor Sealing element for a sensor

1998-06-10

2000-08-01

Tung, T.

Chemistry: electrical and wave energy

Apparatus

Electrolytic

204427, 204428, 277650, 277936, 277939, 277940, 277941, G01N 27407

Patent

active

060961813

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND INFORMATION

The present invention relates to a seal for a sensor element having a ceramic element in the form of a solid electrolyte element embodied as a closed tube and fastened in a sealable manner to a metallic housing. The seal of the present invention is implemented between the solid electrolyte element and the housing.
German Published Patent Application No. 43 42 731 describes sensors based on solid electrolytes in which electrically conductive metal or graphite sealing rings are used for sealed immobilization of the solid electrolyte element in the housing. The metal sealing rings are moreover often covered with a galvanically deposited copper layer. At elevated temperatures, this leads to oxidation and corrosion of the metal or graphite surface. It can moreover cause the metal ions which are thereby created to diffuse into the solid electrolyte or into layers which are arranged on the surface of the solid electrolyte, such as conductive paths, cover layers, and insulation layers. This modifies and impairs their properties in terms of proper function.


SUMMARY OF THE INVENTION

According to the present invention, a sensor is provided that has the advantage that temperature- and corrosion-resistant sealing elements which have a highly ductile roll-clad surface layer of a metal can be used to seal the ceramic element, which may be in the form of a solid electrolyte element embodied as a tubular element. Because of the deformability of the compact seal, the sealing element rests with zero clearance against, in particular, the surface of the ceramic element. This ensures that even under extreme thermal conditions, corrosive substances cannot gain access to the rear portion of the solid electrolyte element and the contacts arranged there, and impair their properties. As compared with conventional galvanically applied metal layers, the advantage of roll-clad metal layers, in addition to their elevated ductility, is that they have a more uniform surface structure, possess no defects, and have a uniform layer thickness. They are moreover much more highly densified, which reduces the surface area for oxygen access. This results in a far lower leakage rate for seals which use sealing rings having roll-clad metal layers than for sealing rings having metal layers which were applied galvanically. A further advantage of roll-clad layers consists in the higher degree of automation in their manufacture as compared with galvanically applied layers and, associated therewith, a continuous process control of the layer thickness. This means that sealing rings produced in this manner have precisely reproducible quality with very close tolerances. As a result, the cost per item, in particular, can be decisively lowered.
An additional roll-clad nickel layer that is applied beneath the copper layer and faces toward the ceramic element allows even greater ductility for the combined copper and nickel layer, which compensates for the surface roughness (R.sub.max as defined by DIN 4768) and shape error of the ceramic element. Partial oxidation of the copper layer is desirable in order to improve the sealing capability of the sealing ring even further. This is achieved by means of a heat pretreatment, preferably at approximately 550 degrees.


BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a longitudinal section through the exhaust-gas portion of a sensor according to the present invention.
FIG. 2 shows an enlarged portion of a sealing zone as depicted in FIG. 1.
FIG. 3 shows a sealing element according to the present invention having a roll-clad copper layer applied to both sides thereof.
FIG. 4 shows the sealing element of the present invention provided with an additional roll-clad nickel layer applied beneath one of the roll-clad copper layers.
FIG. 5 shows the sealing element of the present invention provided with a roll-clad nickel layer applied beneath each of the two roll-clad copper layers.


DETAILED DESCRIPTION

Electrochemical sensor 10 depicted in FIG. 1 has a metal housing 11 which has on its exterio

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R.G. Delagi et al. "High-Aluminum Ferritic Stainless Alloys Synthesized For Catalytic Converters" Advanced Materials and Processes, pp. 27-28 (Jan. 1995).

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