WO1998015819A1

WO1998015819A1 - Sealing element for sensors, and manufacturing process

Info

Publication number: WO1998015819A1
Application number: PCT/DE1997/002012
Authority: WO
Inventors: Karl-Hermann Friese; Helmut Weyl; Hans-Martin Wiedenmann; Anton Hans
Original assignee: Robert Bosch Gmbh
Priority date: 1996-10-10
Filing date: 1997-09-10
Publication date: 1998-04-16
Also published as: EP0865608A1; DE19641808A1; JP2000502456A

Abstract

Disclosed is an electrochemical sensor (10) intended to determine the gas contents in oxygen, including with regard to exhaust gas in internal combustion engines. It ressembles a solid electrolytic body (29) placed in a housing (11) with a sealing ring (21) consisting of various metal layers, the arrangement of which goes from a metal bearing (23) made of an alloy steel. Said bearing (23) is coated on both sides with an aluminium layer (24). One of the aluminium layers (24) can be covered with an additional metal layer (25). During a heat treatment, prior to or after the insertion process, part of the aluminium layer (24) forms with the bearing (23) a Fe-Al alloy with ceramics properties. The aluminium layer (24) surface is oxidised, thereby forming an aluminium oxide layer. Applying an additional metal layer to the aluminium layer results in a mixed oxide layer.

Description

Sealing element for sensors and method for its manufacture

State of the art

The invention relates to a sensor according to the preamble of claim 1. Sensors according to the preamble of claim 1 have a solid electrolyte body which, for. B. executed as a closed tube and tightly fixed in a metallic housing. A seal must be implemented between the solid electrolyte body and the housing.

DE-OS 43 42 731 have sensors

Solid electrolyte base is known, in which electrically conductive metal or graphite sealing rings are used for the tight fixation of the solid electrolyte body in the housing. This type of sealing leads to oxidation and corrosion of the metal or graphite surface at elevated temperatures. Furthermore, it can lead to diffusion of the resulting metal ions into the solid electrolyte. This changes and affects its properties with regard to its trouble-free function. Aluminum-clad steel foils are known from the literature (RG DELAGI & S. IHA, Adv. Mat. Proc. 1995, 27) Treatment form a highly stable Fe / Al alloy and its surface is converted to A1 ₂ 0 ₃ .

Advantages of the invention

The inventive sensor with the characterizing features of the main claim has the advantage that temperature and corrosion-resistant sealing elements can be used to seal the ceramic body, which also have a ductile surface layer made of a light metal or an alloy of a light metal. Due to the deformability of the compact seal, the sealing element lies against the surface of the ceramic body without any gaps. This ensures that even under extreme thermal

Conditions, no corrosive substances get on the ceramic of the solid electrolyte body and impair its properties. The surface coating of the sealing element, which is particularly advantageous for the sealing function, by means of readily deformable aluminum, is irreversibly oxidized in whole or in part after the assembly by thermal treatment and thus additionally acts as an electrical insulator.

The measures listed in the subclaims enable advantageous developments and improvements of the sensor according to the invention and of the method according to the invention. The additional metal layer on the aluminum layer improves the adhesion to the solid electrolyte ceramic. During a thermal treatment, this metal layer oxidizes and forms a stable mixed oxide layer in the form of metal aluminates with the aluminum layer, which is also oxidized. As a result, the metal oxides formed during the oxidation or the metal cations are diffused into sensitive surface layers of the Sensor element hindered. The aluminum layer on the outside is also oxidized to A1 ₂ 0 ₃ and acts as an electrical insulator. The metallic carrier forms a high-temperature and corrosion-resistant Fe-Al alloy with the non-oxidized part of the aluminum. This process advantageously combines the ductility of metals with the toughness and resistance of ceramics and specific alloys.

drawing

Embodiments of the invention are shown in the drawing and explained in more detail in the following description. 1 shows a longitudinal section through the exhaust-side part of a sensor, FIG. 2 shows an enlarged section of a sealing zone according to FIG. 1, FIG. 3 shows a coated sealing ring, FIG. 4 shows the enlarged section of the sealing zone with the sealing element according to FIG. 3 before the thermal treatment and FIG 5 the enlarged section of the sealing zone with a sealing ring according to FIG. 3 after the thermal treatment.

embodiments

The electrochemical measuring sensor 10 shown in FIG. 1 has a metallic housing 11, which has a key hexagon 12 on its outside and a thread 13 as a fastening means for installation in a measuring gas tube, not shown. The housing 11 has a longitudinal bore 17 with a sealing seat 20 which carries a sealing ring 21. On the sealing seat 20 provided with the sealing ring 21 there is a sensor element 14 with a shoulder 16 formed on a bead-shaped head 15. A sealing surface 28 on the sensor element side is formed on the bead-shaped head 15 of the sensor element 14 between the sealing ring 21 and the sensor element 14 out. The sealing seat 20 in turn forms a sealing surface on the house side. The sealing zone 55 which forms on the sealing ring 21 is shown enlarged in FIGS. 2, 4 and 5.

In the present exemplary embodiment, the sensor element 14 is an oxygen probe which is known per se and which is preferably used for measuring the oxygen partial pressure in exhaust gases. The sensor element 14 has a tubular solid electrolyte body 29, the end section on the measuring gas side of which is closed by means of a base 30. On the outside exposed to the measuring gas is a layered, gas-permeable measuring electrode 31 on the solid electrolyte body 29 and on the side facing the interior a reference gas, for. B. air, exposed, gas-permeable and layered reference electrode 32. The measuring electrode 31 is guided with a measuring electrode conductor 33 to a first electrode contact 39 and the reference electrode 32 with a reference electrode conductor 34 to a second electrode contact 40. The electrode contacts 39, 40 are each located on an end face 42 formed by the open end of the solid electrolyte body 29. A porous protective layer 35 is placed over the measuring electrode 31 and partly over the measuring electrode conductor tracks 33. The electrodes 31, 32 and the conductor tracks 27, 28 are advantageously constructed as cermet layers and are co-sintered.

The sensor element 14 protruding from the longitudinal bore 18 of the housing 11 on the measuring gas side is surrounded at a distance by a protective tube 50 which has openings 51 for the entry and exit of the measuring gas and is held at the end of the housing 11 on the measuring gas side. The interior of the sensor element 14 is filled, for example, by a rod-shaped heating element 46, which, not shown, is locked away from the sample gas and is provided with line connections.

A first contact part 44 rests on the first electrode contact 39 and a second contact part 45 rests on the second electrode contact 40. The contact parts 44, 45 are shaped such that they bear against the tubular heating element 40 and are contacted with a measuring electrode connection 47 and a reference electrode connection 48. The connections 47, 48 are not shown

Connection cables contacted and led to the outside to a measuring or control device. In the longitudinal bore 17 of the housing 11, an insulating sleeve 49 is also introduced, which preferably consists of a ceramic material. With the help of a mechanical means, not shown, the insulating sleeve 49 is pressed onto the contact parts 44, 45, as a result of which the electrical connection to the electrode contacts 39 and 40 is produced.

A clear representation of the sealing zone 55 between the

Solid electrolyte body 29 and the housing 11 can be seen in FIG. 2. To protect the conductor track 33, in the region of the sealing surface 28 on the sensor element side, this is covered with an additional protective covering layer 27. The cover layer 27 has a layer thickness of 20 to 100 μm. In the present exemplary embodiment, the cover layer 27 is drawn over the entire area of the conductor track 33 and around the circumference of the solid electrolyte body 29, which is adjacent to the housing 11. However, it is also conceivable to restrict the cover layer 27 only to the area of the sealing surface 28 or to extend the cover layer 27 on the measuring gas side to the protective layer 36, which is advantageous since it causes contamination by soot and / or other conductive deposits from the exhaust gas be avoided. The protective layer 30 consists, for. B. from plasma-sprayed magnesium spinel.

The material of the cover layer 27 is selected so that it withstands the compressive forces of the sealing ring 21, which occur when the sensor element 14 is joined in the housing 11. In addition, it must withstand application temperatures up to 700 ° C in the area of the joint. This is achieved in that a crystalline, non-metallic material forms a load-bearing protective structure in a glaze layer in a homogeneous distribution and the transformation temperature of the glaze is above the application temperature. Possible materials are: A1 ₂ 0 ₃ , Mg spinel, forsterite, MgO stabilized Zr0 ₂ , CaO and / or Y ₂ 0 ₃ stabilized Zr0 ₂ with low stabilizer contents, advantageously with a maximum of 2/3 of the stabilizer oxide of the full stabilization, non-stabilized Zr0 ₂ or Hf0 ₂ or a mixture of these substances. Alkaline earth silicate, for example Ba-Al silicate, is used as the glass-forming material. The Ba-Al silicate, for example, has a thermal one

Expansion coefficient of> 8.5 * 10 ^" K ^" . The barium can be replaced by strontium up to 30% of the atoms.

In order to realize an electrically insulating and gas-tight fastening of the sensor element 14 in the housing 11, the shoulder 16 formed on the bead-shaped head 15 is seated on the housing 11 by means of the sealing ring 21. In order to seal the interior of the sensor element 14, the sealing ring 21 according to FIG. 2 consists of a solid core 23 which forms a support and is made of an iron-chromium or V2A alloy, preferably of Fe-22Cr-MM stainless steel with a thickness of approximately 1.5 mm which is covered on each side by an aluminum layer 24 which is at least 0.01 mm thick. The material is particularly gas, water and fuel impermeable due to its high compression. FIG. 3 shows an exemplary embodiment of the sealing ring 21, which is composed of several different metal layers. Then, for example, an additional metal layer 25 made of a metal, such as copper, nickel, chromium, molybdenum or tungsten, is applied to one of the two aluminum layers 24. The additional metal layer 25 is deposited without current, for example. The thickness of the additional metal layer 25 is, for example, 0.8 μm.

FIG. 4 shows the sealing zone 55 with the metallic sealing ring according to FIG. 3 before the thermal treatment. The additional metal layer 25 lies on the protective layer 27 of the sensor element 14. The second aluminum layer 24 lies against the metallic housing 11.

FIG. 5 shows the sealing zone 55 from FIG. 4 after the thermal treatment. The protective layer 27 of the sensor element 10 remained unchanged. At the interface to the sealing ring 21 according to FIG. 3 there is one

Mixed oxide layer 57 is formed. The layer 58 which follows it consists of an Fe-Al alloy which has ceramic-like properties. The core 23 remained unchanged. Another layer 58 made of the Fe-Al alloy follows above it. Part of the aluminum from layer 24 has converted on the surface to an A10 ₃ layer 59, which at the same time has an electrically insulating effect. In contrast, the metallic housing 11 of the sensor 10 remained unchanged.

It is also possible, for example, to subject the sealing ring 21 to a thermal treatment of approximately 600 ° C. under a protective gas before it is inserted into the housing 11, only the Fe-Al alloy 58 being at least partially formed. The core 23 remains unchanged. After inserting the Solid electrolyte body 29 in the housing 11 by means of the thermally pretreated sealing ring 21, the sensor is heated again to at least 500 ° C., on the sealing ring 21 the surface of the aluminum layer oxidizes to an Al ₂ 0 ₃ layer 59, or the additional metal layer 25 with the mixed oxide layer 57 forms the non-oxidized part of the aluminum layer 24.

Claims

Expectations

1. Sensor, in particular for determining the oxygen content in exhaust gases from internal combustion engines, with a ceramic body (29) which is inserted with a sealing element (21) in a housing (11), characterized in that the sealing element (21) is a metallic carrier (23), which is provided at least on the surface contacting the ceramic body (14, 29) with a light metal layer (24).

2. Sensor according to claim 1, characterized in that the metallic carrier (23) is provided on two opposite sides with the light metal layer (24).

3. Sensor according to claim 1 or 2, characterized in that the layer thickness of the light metal layer (24) corresponds at least to the maximum roughness depth R _max according to DIN 4768 of the light metal layer (24), which is due to the method of applying the light metal layer (24) to the metallic carrier ( 23) can be achieved.

4. Sensor according to claim 1, 2 or 3, characterized in that the layer thickness of the light metal layer (24) is at least 0.01 mm and a maximum of 0.2 mm.

5. Sensor according to one of claims 1 to 4, characterized in that at least on the ceramic body

(29) facing side on the light metal layer (24) an additional metal layer (25) is applied.

6. Sensor according to claim 4, characterized in that the additional metal layer (25) consists of Cu, Ni, Cr, Mo, W or an alloy of these metals.

7. Sensor according to claim 4 or 5, characterized in that the additional metal layer (25) has a layer thickness of <1 microns.

8. Sensor according to claim 1 or 2, characterized in that the metallic carrier (23) made of a corrosion and / or temperature-resistant steel alloy, preferably Fe-Cr, Fe-Cr-V, Fe-Cr-Mo, Fe-Cr- Ni exists.

9. Sensor according to claim 1, 2 or 7, characterized in that the metallic carrier (23) is a sealing ring (21).

10. Sensor according to one of claims 1 to 9, characterized in that the light metal layer made of Mg, Al or an alloy of light metals, preferably made of Al.

11. A method for producing a sensor, in particular for determining the oxygen content in exhaust gases from

Internal combustion engines according to one of claims 1 to 10, characterized in that the coated sealing element (21) is subjected to a thermal treatment.

12. The method according to claim 11, characterized in that after the insertion of the ceramic body (29) in the housing (11) the sensor is subjected to a thermal treatment such that the light metal layer made of aluminum (24) to an aluminum alloy (58) and converted to A1 ₂ 0 ₃ (59).

13. The method according to claim 11, wherein an additional metal layer (25) is applied between the aluminum layer (24) and the metallic carrier (23), characterized in that the aluminum layer (24) and the additional metal layer (25) during the thermal treatment are oxidized and a mixed oxide layer (57) is formed.

14. The method according to any one of claims 11 to 13, characterized in that prior to insertion into the housing (11), the sealing element (21) is subjected to a thermal pretreatment under protective gas, in which the light metal layer (24) with part of the metallic carrier (23) is converted to aluminum alloy (58).

15. The method according to claim 13, characterized in that the additional metal layer (25) is applied in an electroless process.